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.
