Advanced Pilot Training: Engine Gages / Instruments

Manifold Pressure Gage

Manifold pressure gages should be checked before every flight and after every 50 hours of service:

Preflight-Check the instrument by gunning the engine with the supercharger off, if it is of the turbine type. The needle should move freely toward the high side of the scale. Check against barometer, if practical, Be sure manual, automatic, or balance line drain has been operated properly during first warm-up. Drain for 30 seconds at 800 to 1,000 rpm to obtain proper setting.

In the case of any sign of malfunction, all connections should be checked for leaks.

50-Hour-The reading of the instrument should be checked against the station barometer every 50 hours. If the reading differs more than 0.4 inches Hg, the instrument should be removed for a bench test and resetting of the pointer.

Fuel Mixture Indicator

The fuel mixture indicator shows the fuel-to-air ratio and is used as a visual guide for setting the mixture control. When making mixture adjustments, allow at least one minute to elapse before reading the instrument. This will provide time for the change in the carburetor setting to evidence itself.

The fuel-air indicator has a uniform degree of accuracy over a wide range, provided there is no detonation. If there is detonation, the indicated fuelair ratio will be higher than the actual value.

If, as the mixture is leaned out, the indicating needle does not show a leaner mixture, or backs up toward the rich side of the scale, detonation is taking place and the mixture should be made richer.

If, as the mixture control is shifted from position to position, the indicating pointer remains near the "A" point (air point), the instrument is inoperable.

Oil-Pressure Gages

During the operation of aircraft in temperatures below 32'F, engine oil congealing in the oil-pressure gage line may cause the gage to react sluggishly. When this condition exists, the oil-priessure gage line should be drained and refilled with petroleum base hydraulic fluid.

Normally, the fluid will mix with the engine oil very slowly. From 60 to 90 days may elapse before the gage will begin to operate sluggishly again.

The operating limits and ranges of engine instruments and the airspeed indicator are marked on the face of the instruments to facilitate reading these instruments.

The engine instruments are marked with short radial lines in red to indicate limits which should not be exceeded. Operating ranges are shown by green arcs. A white radial line at the bottom of the cover glass is used as an index to show rotation of the glass.

The maximum permissible indicated airspeed is marked with a red line extending from the center of the dial and passing directly over the point corresponding to the maximum permissible indicated airspeed.

REFERENCE: Technical Order 05-1-17, dated November 3, 1943