POWER PLANT (JT8D) -TRIM PROCEDURE
1. General
A. The power plant trim procedure provided here is applicable to all 737 airplanes, and when used with the appropriate set of trim tables, permits proper trimming of any 737 engine configuration. Refer to 71-09-210 for general instructions for use of engine trim data and a trim table locator tabulation.
B. When trimming engines, care must be used to ensure that the correct set of trim tables is being used. The engine model to be trimmed should be determined from the engine nameplate.
C. Power plant trimming is required to adjust idle rpm, check thrust lever cushion and minimize thrust lever stagger. Before a newly installed power plant can be adjusted, it is necessary to depreserve, start, and functionally check the power plant to ensure normal operation (Ref. 71-09-100, Power Plant Operating Procedure). For detailed information on power plant components, indicating systems, and controls refer to the appropriate chapter.
WARNING: ALL PERSONNEL MUST AVOID DANGER AREAS IN FRONT AND REAR OF POWER PLANT DURING GROUND RUNNING OPERATIONS.
CAUTION: BEFORE OPERATING A POWER PLANT, CHECK ENGINE INLET AND AREA IN PROXIMITY OF ENGINE INLET FOR LOOSE OBJECTS AND EQUIPMENT WHICH MAY BE DRAWN INTO ENGINE.
CAUTION: GROUND RUNNING MUST BE KEPT TO A MINIMUM AND PROLONGED RUNNING AT MAXIMUM OPERATING LIMITATIONS AVOIDED. REFER TO "ENGINE STARTING AND OPERATING LIMITATIONS" IN 71-09-100. ENGINE LIFE IS ADVERSELY AFFECTED BY HIGH AND RAPID CHANGES OF ENGINE INTERNAL TEMPERATURES.
D. Engine trimming is performed at part power setting and is a necessary requirement on newly installed engines or following a change of fuel control unit. Periodic trim adjustments may also be necessary during the life of the engine, to restore normal "cushion" between takeoff and full thrust lever position. Since the fuel control is fuel density sensitive, some change will occur in engine trim when switching from one fuel to another. An engine trimmed with Jet B (JP-4) fuel will be "over-trimmed" when operated with Jet A (kerosene) while a Jet A trimmed engine will be "under-trimmed" when operated with Jet B fuel. The amount of throttle cushion available is sufficient to absorb such changes and permit use of either fuel without having to retrim the engines.
However, an engine trimmed with Jet A (high density fuel) will have a slightly lower idle N2 RPM when operated with Jet B (low density fuel), and an engine trimmed with Jet B will have a slightly higher idle N2 RPM when operated with Jet A. To eliminate these effects, it is preferable to trim with the same fuel that will be used in service.
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Aug 15/79 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 71-09-200 Page 201
E. Engine trim is affected by wind velocity and direction. Trim adjustment loses its effectiveness as the angle of wind direction increases toward 90 degrees from the engine inlet. Engines that are trimmed in a crosswind will be "over trimmed" when checked in a no wind condition while engines checked in a head wind will be in an under trimmed condition. Tailwind conditions (beyond 90 degrees) tend to direct engine exhaust gases forward to the engine inlet. Since target values of turbine discharge pressure (Pt7) are corrected for ambient air temperature, exhaust gases ingested by the engine will produce an inaccurate trim adjustment (Fig. 201). Relative wind velocity may be disregarded providing the velocity is under 10 mph and a blast fence is employed. Otherwise, the airplane should be headed into the wind. Engines should not be trimmed when wind velocity exceeds 25 mph or when icing conditions prevail. Engine icing may occur with the following conditions:
(1)
Icing conditions exist when the OAT on the ground and for takeoff, or TAT inflight is 10°C or below, and visible moisture in any form is present (such as clouds, fog with visibility of 1 mile or less, rain, snow, sleet and ice crystals).
(2)
Icing conditions also exist when the OAT on the ground and for takeoff is 10°C or below when operating on ramps, taxiways, or runways where surface snow, ice, standing water, or slush may be ingested by the engines or freeze on engines, nacelles or engine sensor probes.
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