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Aircraft air-cooling differential pressure check.

The air-cooling pressure differential check is a test that checks the effectiveness of the engine's air cooling system.  The test is based on the differential pressure between the pressure side of the cooling and the low pressure (or draft) of the cooling.  For more information see http://www.littleflyers.com/engcool.htm .  A pressure sensing perforated sense hose is temporarily installed in the high pressure area (top of engine most aircraft) and connected by hose to either the pressure side of an airspeed indicator or to the pressure port of a commercial vented gage indicating inches of water.  The gage is displayed in instrument panel area where it can be observed in flight.    A second perforated sense hose is temporary installed in the low pressure area of the engine compartment usually in proximity to the air outlet, but not directly in the draft and is connected to the static port of the air speed indicator or reference port of the commercial gage. .  

Rent the equipment here

Converting airspeed to inches of water

MPH Inches of water
40 0.79
44 1.00
50 1.23
60 1.77
70 2.42
80 3.16
90 4.00
100 4.94
120 7.131
140 9.729
150 11.18

Setting up the equipment

Cowling is removed for access.  Generally the heater ducts are disconnected on both sides of the firewall.  The test gauge hoses are passed through the firewall.  We typically drill a hole in one of the rear baffles and  install a bulkhead fitting (AN832-4D).  The gauge pressure hose is connected the aft side of the fitting and a perforated hose is connected to the front side.  The perforated hose is secured to the top of the engine by use of plastic cable ties).  Another perforated hose is connected to the low pressure reference hose.  The low pressure pressure hose is secured to the engine, mount, cowling, or  airframe structure in proximity of the engine air outlet(s).  The heat ducts are sealed on the firewall with aluminum tape and clamps.  Do not seal off the cabin heat duct from the engine as air will still need to flow through the cabin heat heater even though the duct is disconnected.  The hoses are connected to a differential test gauge.  In the engine compartment the extra hose is secured and bundled.  The cowling is then reinstalled.

Left: Airspeed gauge prepared for differential cooling test.  Right:  The cowling being removed.

Left:  Ducts are removed from the heater valve and the hoses are passed through the firewall.

Left:  A perforated hose.  Right:  A perforated high pressure hose connected to the bulkhead fitting in the baffle.

Left:  The lower pressure hose is secured to the engine mount near the air outlet.  Right:  The heat valve is sealed in the engine compartment.

Left:  The hoses are bundled and secured.  Right:  The cowling is reinstalled.  

The cooling air differential  pressure test

    The aircraft is prepared for normal flight.  Be ready to record the readings or airspeed and differential pressure.  During run-up a small amount of differential pressure should be indicated at 1200 RPM.  On the Cessna 172H tested, 1250 RPM indicated 20 MPH and 1700 indicated 30 MPH.  If you don't achieve any readings during the ground run, check the hoses, baffling, etc..  During the flight, record the airspeed, pressure differential, oil temp, and CHT indications during various flight regimes such as climb-out, fast cruise, descending, cowl flaps open and cowl flaps closed.  .

Best climb indicated 68 MPH

Level Cruise indicated 85 MPH

Shallow climb indicated 74 MPH

500 FPM descent indicated 92 MPH

Final approach 50-60 MPH


Analyzing the data

This 172H doesn't have a serious cooling problem, but it can be improved.  We feel the optimum cooling pressure differential is 110 to 120 MPH (6 to 7 inches of water).

Additional information

Expect the test to require at least 2 hours of labor on a non-pressurized aircraft.

A second follow up article will be written shortly.

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Last modified: February 24, 2007