RUDDER SYSTEM

By Derek Watts & Chris Brady

For the background to the rudder history click here.

Yaw control is achieved by a single graphite / composite rudder panel. A single rudder power control unit (PCU) controls rudder panel deflection. A standby rudder PCU provides back up in the event of malfunction of the main rudder PCU. There is no manual reversion for yaw control. The only internal indication of rudder panel deflection is pedal position, which always accurately reflects control surface deflection. Total authority of the control surface is modulated in relation to aircraft IAS using “blowdown”, ie a constant pressure is applied to the surface by the actuator, and the movement of the panel reduces accordingly as the dynamic pressure on it increases. For this reason maximum rudder pedal movement is reduced with increasing airspeed. Maximum rudder panel deflection is approximately +/-15 degrees on the ground, reducing to around +/-8 degrees at a typical cruise altitude

(PCU)Power Control Unit

The rudder PCU consists of an input shaft / crank mechanism, a dual concentric servo valve to control porting of the fluid to the rudder actuator, and a yaw damper actuator. The rudder actuator is a tandem actuator, having two internal piston areas for each hydraulic source (A & B). The actuator is capable of positioning the rudder panel with either one or both main hydraulic sources available, though with one source inoperative a reduced rudder panel deflection would result due to blowdown at higher airspeeds.
Flow of hydraulic fluid to the rudder actuator is controlled by the dual servo-valve. This is a complex dual concentric cylinder with an outer and inner slide. During normal rudder pedal inputs sufficient rudder panel deflection is catered for by the primary (inner) valve alone. However, should a larger panel deflection be required or a higher rate rudder input be commanded, the secondary (outer) sleeve moves in addition to port extra fluid to the actuator. Movement of the outer sleeve is typically no more than 1mm. Position of both sleeves of the servo valve is controlled by a complex mechanism of bell cranks, input rod and summing lever, the geometry of which is such as to provide movement of the sleeves in relation to the body of the valve.
The 737NG also has a standby rudder PCU that moves the rudder during manual reversion operation. The wheel-torudder interconnect system (WTRIS) will coordinate (assist) turns by using the standby rudder PCU to apply rudder as necessary based upon the Captains control wheel roll inputs. From experience, I can verify that this makes the NG much easier to handle in manual reversion than previous generations

Yaw Damper

The yaw damper is incorporated to prevent Dutch roll. It is connected in parallel with the main servo valve and includes its own actuator, powered by hydraulic system B. This actuator applies its own input to the input shaft / crank mechanism to bring about a movement of the servo-valve and hence a rudder panel deflection. No pedal movement results from yaw damper operation. Total authority of the yaw damper is approximately +/-2.5 degrees.
NGs: The 737-NGs also have a standby yaw damper; it uses the standby rudder PCU, with commands from SMYD 2 and powered by the standby hydraulic system. SMYD 1 controls the main yaw damper with hydraulic system B. Note: Only inputs from the main yaw damper are shown on the yaw damper indicator.
 

Rudder Pressure Reducer (RPR) - 3/4/500

To limit the effects of various PCU failure modes (pre-RSEP), a rudder pressure reducer (RPR) was fitted to the Asystem pressure side of the rudder PCU. This is simply a pressure reducing valve, which operates during the majority of flight phases to reduce the total authority of the rudder panel by approximately one-third. The B-system portion of the rudder PCU is unaffected by the RPR, as is yaw damper operation

During certain critical phases of flight when full rudder authority may be required, the RPR provides full system pressure (3000psi from 1800psi) to the rudder PCU. These are: - 

a)     During take-off below 1000’ Rad.alt.

b)     During approach below 700’ Rad.alt.

c)      If a difference of >45% N1 exists between power units.

d)     If B-system hydraulic pressure is lost.

Whilst correct functioning of the RPR is transparent to the flight crew, certain cockpit indications can be helpful in verifying correct operation and faults alike; the A system flight controls low pressure warning light now has two additional functions related purely to the RPR; on initial application of pressure to hydraulic system A, the lowpressure warning light should remain illuminated for 5 seconds. If the light extinguishes immediately then a fault may be present within the RPR. Incorrect mode switching of the RPR is also indicated by illumination of the light on approach below 700ft RA, indicating that full pressure is not available to the A-system portion of the rudder PCU. A further associated failure of hydraulic system B and an asymmetric thrust condition may result in insufficient rudder authority to maintain directional control.

Digital Yaw Damper Coupler

The yaw damper coupler comprises the control electronics and yaw rate gyro. A digital yaw damper coupler helps reduce the possibility of electro-magnetic interference (EMI). Turn co-ordination is provided by reducing the gain of the yaw-rate gyro in proportion to bank angle detected from the IRU. In this way during a turn the yaw damper coupler is “tricked” into believing the aircraft is yawing into the turn and provides an increased rudder input. The coupler is sensitive only to yaw rates that produce Dutch Roll. Note that the yaw damper coupler controls and monitors both the RPR (Sys A) and RPL (Sys B) of the main rudder PCU, a de-activated yaw damper also renders the RPR & RPL inoperative. For this reason higher block manoeuvring speeds are used when the yaw damper is u/s (not NGs).

Rudder System Enhancement Program (RSEP)

The Rudder System Enhancement Program (RSEP) introduced in 2003 (SB 737-27-1252/3/5) must be implemented on all series of 737s by 12 Nov 2008. It replaces the infamous dual concentric servo valve with separate input rods, control valves and actuators; one set for hydraulic system A, and one set for hydraulic system B. The standby PCU is controlled by a separate input rod and control valve powered by the standby hydraulic system. All three input rods have individual jam override mechanisms that allow inputs to be transferred to the remaining free rods if a jam occurs. All 737s must be fitted with the RSEP by Nov 2008. Modified aircraft are identifiable by the STBY RUD ON light on the flight controls panel and new c/bs on the P6-2 panel labelled “Force Fight Monitor” (all series) and “Rudder Load Limiter” (not NGs).

Force Fight Monitor (FFM)

The main rudder PCU contains a Force Fight Monitor (FFM) that detects opposing pressure (force fight) between A & B actuators, this may happen if either hydraulic system, input rod or control valve has jammed or failed. If this condition is detected for more than 5 seconds, the FFM will automatically turn on the standby hydraulic pump pressurising the standby rudder PCU. This will also illuminate the new STBY RUD ON light on the flight control panel

Lights

From L to R along the panel:

O/B Landing: (Not NG) Three position switch Off - Extend (off) - On. These are located on the outboard flap faring. 

Retractable Landing: (NG only) Replaces the outboard landing lights on the earlier series. These are located on the fuselage just beneath the ram air intakes. The word is that they may be being moved back to their original position on the flap track faring due to excessive stone damage.

Note Use of both of these lights should be avoided at speeds above 250kts due to excessive air loads on their hinges

I/B Landing: Known as fixed landing lights on the NG. Are located in the wing roots, usually used for all day and night landings for conspicuity.

R/W Turnoff: Also in the wing roots, normally only used at night on poorly lit runways.

Taxi: This 250W light is located on the nose gear, on later models it will switch off automatically with gear retraction. It is common practice to have this on whilst the aircraft is in motion as a warning to other aircraft and vehicles.

Logo: Are on each wingtip or horizontal stabiliser and illuminate the fin. Apart from the advertising value on the ground, they are often used for conspicuity in busy airspace.

Position: Depending upon customer option this can be a three position switch (as illustrated) to combine the strobe. Strobe & Steady / Off / Steady, where steady denotes the red, green & white navigation lights. The three Nav lights are no-go items at night.

Strobe: (Not illustrated) Off / Auto / On. Auto is activated by a squat switch. They are also in the wing tips and are very brilliant. This gives rise to great debate amongst pilots about when exactly they should be switched on as they can dazzle other pilots nearby. many people choose to put them on as they enter an active runway for conspicuity against landing traffic.

Anti-Collision: Are the orange rotating beacons above and below the fuselage. They are universally used as a signal that the engines are running or are about to be started. They are typically not switched off until N1 has reduced to below 3.5% (or N2 below 20%) when it is considered safe for ground personnel to approach the aircraft.

Wing: These are mounted in the fuselage and shine down the leading edge of the wing for ice or damage inspection at night.

Wheel Well: Illuminates the main and nose wheel wells. Normally only used during the turnaround at night for the pre-flight inspection but must also be on to see through the gear downlock viewers at night, hence they are a no-go item at night in all but the NG's. There is also a switch for the main wheel well light in the port wheel well.

Water System

There is a 30 US Gal tank (40 US Gal –400 series) behind the aft cargo hold for potable water. This serves the galleys and washbasins, but not the toilets as they use chemicals. Waste water is either drained into the toilet tanks or expelled through heated drain masts.

The tank indicator (-3/4/500 version shown left) is located over the rear service door. Press to test, indications are clockwise from 7 O'clock: Empty, 1/4, 1/2, 3/4, Full. The NG has an LED panel that is always lit (below) for both potable water and waste tank

AIRCRAFT FUSELAGE

EYEBROW WINDOWS

On the 3rd Feb 2005, 737-700, N201LV, L/N 1650, was the first ever 737 to fly without eyebrow windows (window numbers 4 & 5). They have been standard in Boeing aircraft back as far as WW2 bombers to give better crew visibility. Now they have been declared obsolete and removed from production. The design change reduces airplane weight by 20 pounds and eliminates approximately 300 hours of periodic inspections per airplane. Retrofit kits to cover eyebrow windows will be available mid-2006 for the in-service 737 fleet.
Note the windows will still be available as a customer option and all military versions will continue to be delivered with eyebrow windows.
Notice the 10 small vortex generators above the radome. These reduce the cockpit noise from the windshield by 3dB

LAP JOINTS

After the Aloha 737-200 accident, in which a 12ft x 8ft section of the upper fuselage tore away in flight, all 737's with over 50,000 cycles must have their lap joints reinforced with external doublers. This tired old aircraft is a 737-200 and the patching is clearly visable. This modification takes about 15,000 man hours and unfortunately has sometimes been the source of another problem - scoring. This is when metal instruments instead of wooden ones have been used to scrape away excess sealant or old paint from the lap joints which create deep scratches which may themselves develop into cracks.

5 May 2004 - Defects In Aging Passenger Jets Exposed
SEATTLE -- KIRO Team 7 Investigators discover cracks, corrosion and weakened metal hidden inside a growing number of Boeing passenger jets.
The problems lie along structural seams called lap joints. A fuselage is designed with overlapping sheets of metal riveted together. We uncovered at least 28 different warnings regarding flaws or defects. In 2002, a China Airlines jet plummeted into the water, killing 225 passengers. Fourteen years earlier, an Aloha Airlines 737 opened up like a sardine can, killing one person and injuring eight more.
KIRO 7 Eyewitness News Investigative Reporter Chris Halsne discovers a big new problem for Boeing, centered on "lap-joint metal fatigue". The problem is called "scoring". During assembly, workers lay a bead of sealant along this lap joint. It makes the jet more aerodynamic. A year or two flying you around and many jets have to get repainted. Powerful chemical strippers melt the sealant, so some maintenance crews have been putting on caulk then, according to the Federal Aviation Administration, have been cutting away the excess with a box cutter. That can ruin the integrity of the metal along the entire aircraft lap joint. The FAA recently grounded three passenger jets due to "scribe marks" and has identified 32 more Boeing planes with damaging box cutter-type cuts along the lap joint. "When we found this, we jumped on it right away," said FAA spokesperson Mike Fergus. Fergus says they have no idea yet how many more jets are affected by scoring. "With the contraction and expansion of thousands of flight hours, the scratch has the potential, not a guarantee, the potential of turning into a crack. That in turn may have safety factor. That's our issue. If it's safety, we're interested," Fergus said.
Scoring of some lap joints is just the latest chapter in Boeing's long battle with the design and maintenance of its riveted seams. "With that type of structure, whatever is occurring between the two sheets is not readily visible," said Earl Brown, a certified jet engine and airframe mechanic. Brown says the FAA has been warning airlines to inspect -- and re-inspect often -- the lap joints of thousands of still-operating older model Boeing jets. "If we can catch a problem when it's still just a crack and fix it, then we don't have to worry about something coming apart, breaking. The potential for breaking is there if a crack develops. It's pretty much inherent in the design of the airplane and the materials used," Brown said. The scoring issue has been kept quiet until now, but other huge maintenance nightmares include hundreds of previously "patched" or repaired planes.
An Airworthiness Directive says new inspections are necessary to find "premature cracking of certain lap joints, which could result in rapid decompression." Spotting fatigue in the lap joints on the outside of an aircraft, through the paint, is nearly impossible. So here's what the airlines have to do: They have to bring the jet into a hanger and gut the interior. That can cost more than $1 million.
The super-high cost of that "D-check" inspection is hardly an incentive for airlines to look really hard for trouble spots. For example, KIRO Team 7 Investigators uncovered an Aviation Safety Report filed by a mechanic last year. He reported his company ignored a potentially deadly safety problem saying, "A B737-200 had water leaking on passengers and inspectors found all fuselage lap joints leaking excessively." Despite that, the mechanic says the supervisor "told me to get off the ACFT and not to check any laps. This ACFT had to go."
Independent aviation robotics engineer Henry Seemann doesn't look at a Boeing 737 like the rest of us. We view them as a whole. He sees them in tiny parts, up close, one rivet at a time. And what he sees should make all of us a little nervous: cascading metal cracks, loose shear clips, corroded lap joints and tiny cuts in the metal. Halsne: "Are there times when you walk up to a plane and think, 'I don't know about this one?'" Seemann: "Yes, I've had my moments of certain airplanes when I've looked at them and actually booked a different flight." Seemann invented a machine, currently used by Boeing itself, that automatically inspects lap joints. The robot could save the industry billions in early maintenance because it takes just a few days to computer map and analyze lap joint flaws. Current methods take a month.
Despite the potential cost savings some airlines are telling Henry don't get that thing near our passenger jets. "There's a requirement that if you know something is wrong with your airplane, you're supposed to fix it. It's a moral thing," Seemann said. "Some are afraid of that -- that their fleet is kind of old and we're going to inspect their planes and we're going to put a big red "x" on them." The Federal Aviation Administration confirms this robot design is in the final stages of approval. It could revolutionize the way we spot catastrophic metal failures - before a serious accident.
Boeing refused our repeated requests for an on-camera interview about "scoring" and other lap joint issues, but did provide us with some background on how it's working hard to fix the problems. We called Boeing again this week for a statement. While they still won't comment on past metal fatigue issues, they did say design improvements on their new line of 7E7's should take care of future problems

RADOME

The radome (RAdar DOME) is an aerodynamic faring that houses the weather radar and ILS localiser and glideslope antennas. Unlike the rest of the fuselage it is made of fibreglass to allow the RF signals through.
Fibreglass is non-conductive which would allow the build up of P-static (static due to the motion of the aircraft through precipitation). This would in turn cause static interference on the antenna within so the radome is fitted with six conductive diverter strips on the outside to dissipate P-static into the airframe.

AFT BODY VORTEX GENERATORS


There are four vortex generators on each side of the rear fuselage above the horizontal stabiliser. The 737-1/200s also had three vortex generators below each stabiliser. They were probably installed to energise the airflow at the stagnation point at the tailcone, thereby reducing drag and giving a slight performance advantage.
Classics were initially produced without any aft body vortex generators (see photo. However the upper vortex generators were reinstated after line number 2277 (May 1992 onwards). This was to reduce elevator and elevator tab vibration during flight to increase their hinge bearing service life.
The CDL says that if any of these vortex generators are not fitted or missing “occasional vertical motions may be felt which appear to be light turbulence These motions are characteristic of this airplane and should not be construed to be associated with Mach buffet.”

technical Description of rudder

RUDDER SYSTEM

Yaw control is achieved by a single graphite / composite rudder panel. A single rudder power control unit (PCU) controls rudder panel deflection. A standby rudder PCU provides back up in the event of malfunction of the main rudder PCU. There is no manual reversion for yaw control. The only internal indication of rudder panel deflection is pedal position, which always accurately reflects control surface deflection. Total authority of the control surface is modulated in relation to aircraft IAS using “blowdown”, ie a constant pressure is applied to the surface by the actuator, and the movement of the panel reduces accordingly as the dynamic pressure on it increases. For this reason maximum rudder pedal movement is reduced with increasing airspeed. Maximum rudder panel deflection is approximately +/-15 degrees on the ground, reducing to around +/-8 degrees at a typical cruise altitude.


Power Control Unit (PCU)

The rudder PCU consists of an input shaft / crank mechanism, a dual concentric servo valve to control porting of the fluid to the rudder actuator, and a yaw damper actuator. The rudder actuator is a tandem actuator, having two internal piston areas for each hydraulic source (A & B). The actuator is capable of positioning the rudder panel with either one or both main hydraulic sources available, though with one source inoperative a reduced rudder panel deflection would result due to blowdown at higher airspeeds.
Flow of hydraulic fluid to the rudder actuator is controlled by the dual servo-valve. This is a complex dual concentric cylinder with an outer and inner slide. During normal rudder pedal inputs sufficient rudder panel deflection is catered for by the primary (inner) valve alone. However, should a larger panel deflection be required or a higher rate rudder input be commanded, the secondary (outer) sleeve moves in addition to port extra fluid to the actuator. Movement of the outer sleeve is typically no more than 1mm. Position of both sleeves of the servo valve is controlled by a complex mechanism of bell cranks, input rod and summing lever, the geometry of which is such as to provide movement of the sleeves in relation to the body of the valve.
The 737NG also has a standby rudder PCU that moves the rudder during manual reversion operation. The wheel-torudder interconnect system (WTRIS) will coordinate (assist) turns by using the standby rudder PCU to apply rudder as necessary based upon the Captains control wheel roll inputs. From experience, I can verify that this makes the NG much easier to handle in manual reversion than previous generations.
 
 

YAW DAMPER


The yaw damper is incorporated to prevent Dutch roll. It is connected in parallel with the main servo valve and includes its own actuator, powered by hydraulic system B. This actuator applies its own input to the input shaft / crank mechanism to bring about a movement of the servo-valve and hence a rudder panel deflection. No pedal movement results from yaw damper operation. Total authority of the yaw damper is approximately +/-2.5 degrees.
NGs: The 737-NGs also have a standby yaw damper; it uses the standby rudder PCU, with commands from SMYD 2 and powered by the standby hydraulic system. SMYD 1 controls the main yaw damper with hydraulic system B. Note: Only inputs from the main yaw damper are shown on the yaw damper indicator.

 

Rudder Pressure Reducer (RPR) - 3/4/500

To limit the effects of various PCU failure modes (pre-RSEP), a rudder pressure reducer (RPR) was fitted to the Asystem pressure side of the rudder PCU. This is simply a pressure reducing valve, which operates during the majority of flight phases to reduce the total authority of the rudder panel by approximately one-third. The B-system portion of the rudder PCU is unaffected by the RPR, as is yaw damper operation

During certain critical phases of flight when full rudder authority may be required, the RPR provides full system pressure (3000psi from 1800psi) to the rudder PCU. These are: - 

a)     During take-off below 1000’ Rad.alt.

b)     During approach below 700’ Rad.alt.

c)      If a difference of >45% N1 exists between power units.

d)     If B-system hydraulic pressure is lost.

Whilst correct functioning of the RPR is transparent to the flight crew, certain cockpit indications can be helpful in verifying correct operation and faults alike; the A system flight controls low pressure warning light now has two additional functions related purely to the RPR; on initial application of pressure to hydraulic system A, the lowpressure warning light should remain illuminated for 5 seconds. If the light extinguishes immediately then a fault may be present within the RPR. Incorrect mode switching of the RPR is also indicated by illumination of the light on approach below 700ft RA, indicating that full pressure is not available to the A-system portion of the rudder PCU. A further associated failure of hydraulic system B and an asymmetric thrust condition may result in insufficient rudder authority to maintain directional control

Digital Yaw Damper Coupler

The yaw damper coupler comprises the control electronics and yaw rate gyro. A digital yaw damper coupler helps reduce the possibility of electro-magnetic interference (EMI). Turn co-ordination is provided by reducing the gain of the yaw-rate gyro in proportion to bank angle detected from the IRU. In this way during a turn the yaw damper coupler is “tricked” into believing the aircraft is yawing into the turn and provides an increased rudder input. The coupler is sensitive only to yaw rates that produce Dutch Roll. Note that the yaw damper coupler controls and monitors both the RPR (Sys A) and RPL (Sys B) of the main rudder PCU, a de-activated yaw damper also renders the RPR & RPL inoperative. For this reason higher block manoeuvring speeds are used when the yaw damper is u/s (not NGs).

Rudder System Enhancement Program (RSEP)

The Rudder System Enhancement Program (RSEP) introduced in 2003 (SB 737-27-1252/3/5) must be implemented on all series of 737s by 12 Nov 2008. It replaces the infamous dual concentric servo valve with separate input rods, control valves and actuators; one set for hydraulic system A, and one set for hydraulic system B. The standby PCU is controlled by a separate input rod and control valve powered by the standby hydraulic system. All three input rods have individual jam override mechanisms that allow inputs to be transferred to the remaining free rods if a jam occurs. All 737s must be fitted with the RSEP by Nov 2008. Modified aircraft are identifiable by the STBY RUD ON light on the flight controls panel and new c/bs on the P6-2 panel labelled “Force Fight Monitor” (all series) and “Rudder Load Limiter” (not NGs).

Force Fight Monitor (FFM)

The main rudder PCU contains a Force Fight Monitor (FFM) that detects opposing pressure (force fight) between A & B actuators, this may happen if either hydraulic system, input rod or control valve has jammed or failed. If this condition is detected for more than 5 seconds, the FFM will automatically turn on the standby hydraulic pump pressurising the standby rudder PCU. This will also illuminate the new STBY RUD ON light on the flight control panel

Rudder Pressure Limiter (Not NGs)

This is effectively the B system equivalent of the RPR, except that it is physically part of the main rudder PCU rather than upstream of it. Hydraulic system B pressure is reduced within the main PCU from 3000 to 2200psi it has the same activation criteria as the RPR

NGs

The NGs do not have an RPR or RPL, but two Load Limiters instead (Shown as “CONTROL VALVE”s in the FCOM schematics). At speeds above approximately 135kts, hydraulic system A pressure (Pre-RSEP), hydraulic system A and B pressure (Post-RSEP) to the rudder PCU is reduced to 1450psi (Pre-RSEP) / 2200psi (Post-RSEP). They both reduce rudder output force by 25% at blowdown speed. The NGs also gained the FFM and separate input rods, control valves and actuators of the RSEP package.

Analysis of QRH Procedures

The QRH procedures for rudder malfunctions were first introduced in January 1997. Since then they have changed several times, either to simplify the procedures or as a result of hardware changes. They still have many branches depending upon the RSEP status of the aircraft and what condition is detected by the crew.
An uncommanded rudder deflection and/or hardover may be caused by a number of different failure modes within the rudder PCU and/or yaw damper, and the severity of symptoms could differ widely from a nuisance yaw damper deflection to a full-scale rudder hardover resulting in reduced controllability. As identification of the primary cause of such a failure may be impossible in flight, certain procedures have been mandated which aim to minimize the effects of the malfunction. There are two similar QRH drills which cover this situation:

JAMMED OR RESTRICTED FLIGHT CONTROLS
Condition: Movement of the elevator, aileron/spoiler or rudder is restricted
UNCOMMANDED RUDDER/YAW OR ROLL
Condition: Uncommanded rudder pedal displacement or pedal kicks or uncommanded yaw or roll.

The latter is more appropriate for a rudder hardover, but either procedure will direct you to the correct solution.

AUTOPILOT (if engaged)…………………..DISENGAGE
AUTOTHROTTLE (if engaged)……………DISENGAGE
Verify thrust is symmetrical.
These are the only recall items. First disengage the automatics, get control of the flight path and verify thrust is symmetrical. If you have a STBY RUD ON light installed (and serviceable), ie an RSEP a/c, then go to the JAMMED OR RESTRICTED FLIGHT CONTROLS checklist. The logic here is that the FFM will either have detected an opposing pressure between A & B actuators and activated the standby rudder PCU or there was no opposing pressure and the problem was a jam rather than a hydraulic problem.
The rest of this section assumes that you do not have a STBY RUD ON light installed (ie Pre-RSEP a/c).
YAW DAMPER……………………………..OFF
The yaw damper is switched OFF. This removes power to the yaw damper actuator, therefore eliminating any input to the main rudder PCU. Whilst this should eliminate any nuisance yaw and secondary roll caused by a failure within the yaw damper or coupler, with its limited authority over main rudder panel deflection it is highly questionable whether this alone could produce a large-scale uncommanded rudder movement.
If the yaw or roll is the result of uncommanded rudder displacement or pedal kicks:
Rudder trim………………………………….Center
Rudder pedals………………………………..Free & center
After verifying zero rudder trim, the checklist calls for a maximum combined effort of both pilots to centre the rudder pedals. The intention of this is to provide a maximum force to shear any metal fragments or “chips” which may be present within the servo-valve. Remember, centred pedals mean a centralised rudder.
If rudder pedal position and/or movement are not normal:
SYSTEM B FLIGHT CONTROL switch….STBY RUD
During a normal flight phase, the rudder has three separate sources of power; A-system hydraulics, B-system hydraulics and Standby hydraulics. The objective of any such drill would be to reduce the uncommanded deflection of the rudder panel, and to restore some directional controllability. This is achieved within the drill by first reducing the authority of the main rudder PCU by removing the B-system hydraulic source. A-system pressure remains, but at a considerably reduced pressure due to the functioning of the RPR / load limiter, hence blowdown will help to realign the control surface. Re-positioning the B-system flight controls switch to standby rudder removes B-system hydraulic pressure from the main rudder PCU, and provides a completely independent method of rudder panel control through the standby rudder PCU, further assisting in re-alignment of the control surface.

BOEING 737 FLIGHT DECK

In the NG, the larger PFD/ND (formerly known as EFIS/MAP) are now side by side to fit into the space available, controls for these are located either side of the MCP. The EIS & fuel gauges are both on the central CDS with a sixth screen below that, between the CDU's. The flat panel displays have the advantage over CRT's of being lighter, more reliable and consume less power, although they are more expensive to produce.

The overhead panel remains largely unchanged since the -100, apart from a digital AC & DC metering panel & DCPCS.
According to Boeing, the requirements from the airlines for the new cockpit were:

• To be easy for current 737 pilots to operate.
• To anticipate future requirements eg transitioning to 777 style flightdecks.
• To accommodate emerging navigation and communication technologies.

For an in-depth look at the NG flightdeck, follow this link to the article in Aero No.04 from Boeing

727  |  737-100  |  737-200  |  737-3/4/500

Photos of Boeing 737 Engine

The left hand side of the CFM56-3. The large silver coloured pipe is the start air manifold with the starter located at its base. The black unit below that is the CSD. The green unit forward (left) of the CSD is the generator cooling air collector shroud, the silver-gold thing forward of that (with the wire bundle visible) is the generator, and the green cap most forward is the generator cooling air inlet.

The view into the JT8D jetpipe.

The corrugated ring is the mixer unit, this is designed to thoroughly mix the bypass air with the turbine exhaust.

The exhaust cone makes a divergent flow which slows down the exhaust and also protects the rear face of the last turbine stage.

The view into the CFM56-3 jetpipe.
This is the turbine exhaust area, no mixing is required as the bypass air is exhausted coaxially

There are two fan inlet temperature sensors in the CFM56-3 engine intake. The one at the 2 o'clock position is used by the PMC and the one at the 11 o'clock position is used by the MEC. The MEC uses the signal to establish parameters to control low and high idle power schedules.

The temperature data is used for thrust management and variable bleed valves, variable stator vanes & high / low pressure turbine clearance control systems.

The CFM56-7 inlet has just one fan inlet temperature probe, which is for the EEC (because there is no PMC on the NG's).
A subtle difference between the NG & classic temp probes is that the NG's only use inlet temp data on the ground and for 5 minutes after take-off. In-flight after 5 minutes temp data is taken from the ADIRU's.
The temperature data is used for thrust management and variable bleed valves, variable stator vanes & high / low pressure turbine clearance control systems

The CFM56-7 spinner has a unique conelliptical profile. The first 737-3/400's had a conical (sharp pointed) spinner but these tended to shed ice into the core. This was one of the reasons for the early limitation of minimum 45% N1 in icing conditions which made descent management quite difficult. They were later replaced with elliptical (round nosed) spinners which succeeded in deflecting the ice away from the core, but because of their larger stagnation point, were more prone to picking up ice in the first place. The conelliptical spinner of the NG's neatly solves both problems

The CFM56-7 tailpipe is slightly longer then the CFM56-3 and has a small tube protruding from the faring. This is the Aft Fairing Drain Tube for any hydraulic fluid, oil or fuel that may collect in there. There is also a second drain tube that does not protrude located on the inside of the fairing.

The JT8D tailpipe fitted as standard from l/n 135 onwards.
The original thrust reversers were totally redesigned by Boeing and Rohr since the aircraft had inherited the same internal pneumatically powered clamshell thrust reversers as the 727 which were relatively ineffective and apparently tended to lift the aircraft off the runway when deployed! The redesign to external hydraulically powered target reversers cost Boeing $24 million but dramatically improved its short field performance which boosted sales to carriers proposing to use the aircraft as a regional jet from short runways. Also the engine nacelles were extended by 1.14m as a drag reduction measure

The outboard side of the JT8D-9A with the cowling open.

Reverse Thrust

he original 737-1/200 thrust reversers were pneumatically powered clamshell doors taken straight from the 727 (shown left). When reverse was selected, 13th stage bleed air was ported to a pneumatic actuator that rotated the deflector doors and clamshell doors into position. Unfortunately they were relatively ineffective and apparently tended to push the aircraft up off the runway when deployed. This reduced the downforce on the main wheels thereby reducing the effectiveness of the wheel brakes.

By 1969 these had been changed by Boeing and Rohr to the much more successful hydraulically powered target type thrust reversers (shown right). This required a 48 inch extension to the tailpipe to accommodate the two cylindrical deflector doors which were mounted on a four bar linkage system and associated hydraulics. The doors are set 35 degrees away from the vertical to allow the exhaust to be deflected inboard and over the wings and outboard and under the wings. This ensures that exhaust and debris is not blown into the wheel-well, nor is it blown directly downwards which would lift the weight off the wheels or be re-ingested. Fortunately the new longer nacelle improved cruise performance by improving internal airflow within the engine and also reduced cruise drag. These thrust reversers are locked against inadvertent deployment by both deflector door locks and the four bar linkage being overcenter. To illustrate how poor the original clamshell system was, Boeings own data says target type thrust reversers at 1.5 EPR are twice as effective as clamshells at full thrust!

The CFM56 uses blocker doors and cascade vanes to direct fan air forwards. Net reverse thrust is defined as: fan reverser air, minus forward thrust from engine core, plus form drag from the blocker door. As this is significantly greater at higher thrust, reverse thrust should be used immediately after landing or RTO and, if conditions allow, should be reduced to idle by 60kts to avoid debris ingestion damage. Caution: It is possible to deploy reverse thrust when either Rad Alt is below 10ft – this is not recommended

The REVERSER light shows either control valve or sleeve position disagreement or that the auto-restow circuit is activated. This light will illuminate every time the reverser is commanded to stow, but extinguishes after the stow has completed, and will only bring up master caution ENG if a malfunction has occurred. Recycling the reverse thrust will often clear the fault. If this occurs in-flight, reverse thrust will be available after landing.

The REVERSER UNLOCKED light (EIS panel) is potentially much more serious and will illuminate in-flight if a sleeve has mechanically unlocked. Follow the QRH drill, but only multiple failures will allow the engine to go into reverse thrust.

The 737-1/200 thrust reverser panel has a LOW PRESSURE light which refers to the reverser accumulator pressure when insufficient pressure is available to deploy the reversers. The blue caption between the switches is ISOLATION VALVE and illuminates when the three conditions for reverse thrust are satisfied: Engine running, Aircraft on ground & Fire switches in normal position. The guarded NORMAL / OVERRIDE switches to enable the reverse thrust to be selected on the ground with the engines stopped (for maintenance purposes).

Huskit

The first "hushkit" was not visible externally, in 1982 exhaust mixers were made available for the JT8D-15, -17 or -17R. These were fitted behind the LP turbine to mix the hot gas core airflow with the cooler bypassed fan air. This increased mixing reduced noise levels by up to 3.6 EPNdB.
Several different Stage III hushkits have been available from manufacturers Nordam (shown right) and AvAero since 1992. The Nordam comes in HGW and LGW versions.
As hushkits use more fuel, the EU tried to ban all hushkitted aircraft flying into the EU from April 2002. This was strongly opposed and the directive has been changed to allow hushkitted aircraft to use airports which will accept them.
737 classics may be fitted with hardwall forward acoustic panels which reduce noise by 1 EPNdB

Engine Starting

Engine Starting

Min duct pressure for start (Classics only): 30psi at msl, -½psi per 1000ft pressure altitude. Max: 48psi. 
Min 25% N2 (or 20% N2 at max motoring) to introduce fuel; any sooner could result in a hot start. Max motoring is when N2 does not increase by more than 1% in 5 seconds. 
Aborted engine start criteria:

• No N1 (before start lever is raised to idle).
• No oil pressure (by the time the engine is stable).
• No EGT (within 10 secs of start lever being raised to idle).
• No increase, or very slow increase, in N1 or N2 (after EGT indication).
• EGT rapidly approaching or exceeding 725˚C.

An abnormal start advisory does not by itself mean that you have to abort the engine start.
Starter cutout is approx 46% N2 -3/4/500; 56% N2 -NG's.
Starter duty cycle is:

• First attempt: 2mins on, 20sec off.
• Second and subsequent attempts: 2mins on, 3mins off.

Do not re-engage engine start switch until N2 is below 20%.

During cold weather starts, oil pressure may temporary exceed the green band or may not show any increase until oil temperature rises. No indication of oil pressure by the time idle RPM is achieved requires an immediate engine shutdown. At low ambient temperatures, a temporary high oil pressure above the green band may be tolerated.

When starting the engines in tailwind conditions, Boeing recommends making a normal start. Expect a longer cranking time to ensure N1 is rotating in the correct direction before moving the start lever. A higher than normal EGT should be expected, yet the same limits and procedures should apply.

Upper DU in Compact Display mode

The Compact Display mode can only be shown when the MFD ENG button is pressed for the first time after the aircraft has been completely shut down. The photo shows this display with one engine started and nicely illustrates the blank parameters which are controlled by the EEC and hence are only displayed when the EEC powers up when the associated start switch is selected to GND. During start-up the EEC's receive electrical power from the AC transfer busses, but their normal source of power are their own alternators which cut-in when N2 is above 15%.

Ignition

Ignition

There are two independent AC ignition systems, L & R. Starting with R selected on the first flight of the day provides a check of the AC standby bus, which would be your only electrical source with the loss of thrust on both engines and no APU. Normally, in-flight, no igniters are in use as the combustion is self-sustaining. During engine start or take-off & landing, GND & CONT use the selected igniters. In conditions of moderate or severe precipitation, turbulence or icing, or for an in-flight relight, FLT should be selected to use both igniters. NG aircraft: for in-flight engine starts, GRD arms both igniters.

The 737-NG's allow the EEC to switch the ignition ON or OFF under certain conditions:

• ON: For flameout protection. The EEC will automatically switch on both ignition systems if a flameout is detected.
• OFF: For ground start protection. The EEC will automatically switch off both ignition systems if a hot or wet start is detected.

Note that older 737-200s have ignition switch positions named GRD, OFF, L IGN, R IGN and FLT while newer 737s use GRD, OFF, CONT and FLT. This is why QRH uses "ON" (eg in the One Engine Inop Landing checklist) to cover both LOW IGN & CONT for operators with mixed fleets consisting of old and new versions of the 737.

737-200 Ignition panel

Oil

Oil

Oil pressure is measured before the bearings, where you need it; oil temperature on return, at its hottest; and oil quantity at the tank, which drops after engine start. Oil pressure is unregulated, therefore the yellow band (13-26psi) is only valid at take-off thrust whereas the lower red line (13psi) is valid at all times. If the oil pressure is ever at or below the red line, the LOW OIL PRESSURE light will illuminate and that engine should be shut down. NB on the 737-1/200 when the oil quantity gauge reads zero, there could still be up to 5 quarts present

Fuel

Fuel

Thrust (fuel flow) is controlled primarily by a hydro-mechanical MEC in response to thrust lever movement, as fitted to the original 737-1/200’s. In the –3/4/500 series, fuel flow is further refined electronically by the PMC, which acts without thrust lever movement. The 737-NG models go one stage further with FADEC (EEC).

The 3/4/500's may be flown with PMC’s inoperative, but an RTOW penalty (ie N1 reduction) is imposed because the N1 section will increase by approximately 4% during take-off due to windmilling effects (FOTB 737-1, Jan 1985). This reduction should save reaching any engine limits. The thrust levers should not be re-adjusted during the take-off after thrust is set unless a red-line limit is likely to be exceeded, ie you should allow the N1's to windmill up.

Fuel is heated to avoid icing by the returning oil in the MEC.

Tech Insertion

Tech Insertion

"Tech Insertion" is an upgrade to the CFM56-5B & 7B available from early 2007. The package includes improvements to the HP compressor, combustor and HP & LP turbines. The package give a longer time on wing, about 5% lower maintenance costs, 15-20% lower oxides of nitrogen (NOx) emissions, and 1% lower fuel burn.

Tech Insertion will become the new production configuration for both the CFM56-7B and CFM56-5B. CFM is also defining potential upgrade kits that could be made available to operators by late 2007.

CFM56-7BE "Evolution"

The new CFM56-7BE Product Improvement package announced in 2009 will have the following design changes & improvements:

• HPC outlet guide vane diffuser area ratio improved and pressure losses reduced.
• HPT blades numbers reduced, axial chord increased, tip geometry improved. Rotor redesigned.
• LPT blade & vane numbers reduced and profiles based on optimized loading distribution. LEAP56 incorporated.
• Primary nozzle, plug & strut faring all redesigned.

The -7BE will be able to be intermixed with regular SAC/DAC or Tech Insertion engines subject to updated FMC, MEDB and EEC. Entry into service is planned for mid-2011

From the press 2 Aug 2010:

CFM International has won certification for its upgraded CFM56-7BE engine from the FAA and the European Aviation Safety Agency (EASA), and is working with Boeing to prepare for flight tests on a Boeing 737 starting in the fourth quarter of this year.

Entry into service is planned for mid-2011 to coincide with 737 airframe improvements that, together with the engine upgrade, are designed to provide a 2% improvement in fuel consumption. CFM provisionally scheduled engine certification by the end of the third quarter, but says development, including recently completed flight tests, have progressed faster than expected. Improvements include a new high-pressure compressor outlet guide vane diffuser, high-pressure turbine blades, disks and forward outer seal. The package also includes a new design of low-pressure turbine blades, vanes and disk.

The first full CFM56-7BE type design engine completed ground testing in January 2010, and overall completed 390 hours of ground testing, says the Franco-U.S. engine maker. In addition, the upgraded CFM completed a 60-hour certification flight test program in May on GE’s modified 747 flying testbed in Victorville, Calif.

At the recent Farnborough International Airshow, company officials said discussions are continuing with Airbus about a possible upgrade for the CFM56-5B for the A320 family based on the same technology suite. A decision on whether or not an upgraded variant will be developed for Airbus will be finalized by year-end, adds the engine maker.

Power Plant History

History

The original choice of powerplant was the Pratt & Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.

Auxiliary inlet doors were fitted to early JT8D's around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.

The sole powerplant for all 737's after the -200 is the CFM-56. The core is produced by General Electric and is virtually identical to the F101 as used in the Rockwell B-1. SNECMA produce the fan, IP compressor, LP turbine, thrust reversers and all external accessories. The name "CFM" comes from GE's commercial engine designation "CF" and SNECMA's "M" for Moteurs.

One problem with such a high bypass engine was its physical size and ground clearance; this was overcome by mounting the accessories on the lower sides to flatten the nacelle bottom and intake lip to give the "hamster pouch" look. The engines were moved forward and raised, level with the upper surface of the wing and tilted 5 degrees up which not only helped the ground clearance but also directed the exhaust downwards which reduced the effects of pylon overheating and gave some vectored thrust to assist take-off performance. The CFM56-3 proved to be almost 20% more efficient than the JT8D.

The NG's use the CFM56-7B which has a 61 inch diameter solid titanium wide-chord fan, new LP turbine turbomachinery, FADEC, and new single crystal material in the HP turbine. All of which give an 8% fuel reduction, 15% maintenance cost reduction and greater EGT margin compared to the CFM56-3.

One of the most significant improvements in the powerplant has been to the noise levels. The original JT8D-9 engines in 1967 produced 75 decibel levels, enough to disrupt normal conversation indoors, within a noise contour that extended 12 miles along the take-off flight path. Since 1997 with the introduction of the 737-700’s CFM56-7B engines, the 75-decibel noise contour is now only 3.5 miles long.

The core engine (N2) is governed by metering fuel (see below), whereas the fan (N1) is a free turbine. The advantages of this include: minimised inter-stage bleeding, fewer stalls or surges and an increased compression ratio without decreasing efficiency.

This quote from CFMI in 1997:

 

"Since entering service in 1984, the CFM56-3 has established itself as the standard against which all other engines are judged in terms of reliability, durability, and cost of ownership. The fleet of nearly 1,800 CFM56-3-powered 737s in service worldwide have logged more than 61 million hours and 44 million cycles while maintaining a 99.98 percent dispatch reliability rate (one flight delayed or cancelled for engine-caused reasons per 5,000 departures), a .070 shop visit rate (one unscheduled shop visit per 14,286 flight hours), and an in-flight shutdown rate of .003 (one incident per 333,333 hours)."