FLYING THE ENGINE Preflighting Your Engine

The TPE331 engine that powers all turboprop Twin Commanders has been around for decades, but the operating principles that guided its design—simplicity, power, fuel efficiency, and safety—are as relevant today as when the first production engine appeared more than 45 years ago.

The following article, which is adapted from Honeywell’s TPE331 Pilot Tips booklet, takes a look at preflight inspection procedures.

The importance of a thorough preflight inspection by a flight crewmember cannot be overemphasized. Remember, in some cases it will be necessary to use a stepladder to adequately examine the engine inlet area.

  • Cleared/Deferred write-ups—checked
  • GPU (if use is intended)—check operation

If external power is being used for engine start, proper operation and setting, such as adequate fuel (internal combustion-powered GPU), appropriate voltage and amperage (28 volt/800-1600 amps) is of great importance. Consult the AFM/POH for the appropriate electric rating when using a GPU for engine starting or systems checks.

  • Engine inlet/exhaust cover—removed
  • Engine cowling—inspect security
  • Oil level and filler cap—check level and security

If the engine has not been operated for several hours, the oil level should be checked prior to starting; however, care should be taken to avoid overfilling. Occasionally, oil may be trapped within the engine gear case and will not show on the sight gauge or dipstick. To assure a valid oil level check, the engine should first be motored up to 15 percent, or with a low battery the propeller should be pulled through by hand to cause residual oil in the gearbox to be scavenged back to the oil tank. The best time to check the correct oil level is within one hour after shutdown when oil is distributed throughout the engine as it is during operation, and near operating temperatures. Visually confirm a clear propeller before engaging the starter or before motoring the propeller. Also, exercise extreme care when opening oil tank dipstick cap immediately following engine shutdown because hot oil can spill and cause injury.

  • Oil/fuel filter bypass valves—check indicators

Oil filter bypass: An extended red pin or poppet indicates a restricted oil filter element. However, in very cold weather, due to increased oil viscosity, delta pressure across the filter element could exceed bypass filter values, causing momentary opening of the bypass valve. Redesigned bypass valves (if incorporated) provide a thermal lockout, preventing bypass indicator extension at oil temperature below about 38 degrees C.

Fuel filter bypass: Some installations have a pin or poppet that extends to indicate that the fuel filter bypass valve has opened. Other installations have pressure ports on the bypass valve, causing a cockpit light to illuminate whenever restricted fuel flow is encountered, indicating an impending filter bypass condition.

  • Fuel drains—check as per AFM/POH
  • Oil cooler air inlet—check. Should be clean, unobstructed, no evidence of leaks
  • Propeller blades—check (on the start locks)

On the ground only, prior to engine starting, verify that the prop is on the locks (flat pitch, 1-2 degrees). If the blades are in feather (85-90 degrees), move and hold the power lever in the full reverse position and then use the unfeathering pump. Remember that oil from the oil tank is used to pump the prop blades toward the start locks position. In other words, unfeathering will result in the transfer of oil from the oil tank to the gear case. Therefore, subsequent oil level check may be erroneous. (See oil level and filler cap check procedures above).

  • Propeller blades—condition

Check the leading edges of the propeller for erosion, nicks, cracks and/or bent blades. Any of these discrepancies can become worse over time and may cause propeller imbalance. Lack of propeller balance may impact engine wear.

  • Propeller hub/spinner—check

Check for security and oil/grease leaks. Improper servicing can cause grease to leak, creating propeller imbalance.

  • Engine inlet—check sensors.

Damaged or blocked sensors can send erroneous signals to the FCU/SRL and can cause erratic engine operations. The inlet should be clear and unobstructed. Check inlet surface for discoloration (possibly due to excessive use of inlet heat during ground operation) and for evidence of residual oil. Minor compressor seal leaks are typically a nuisance and do not normally affect the airworthiness of the engine. However, the leak should be written up and brought to the prompt attention of your maintenance department or service facility.

  • Tt2 Sensor (SRL/VRL)–check

The P2-T2 sensor (T2 only with Bendix FCU) should be checked for security and to assure that they are undamaged and clean. If located in the inlet, the Tt2 sensor can be found opposite of the P2T2 sensor, in the oil cooler inlet or anywhere on the engine cowling, depending on the aircraft type. The Tt2 sensor provides temperature-sensing information to the SRL controller on Dash 10 engines. The sensor should be checked for general conditions and security.

  • Engine inlet/1st stage—check compressor

Inspect the entire 360 degrees of the visible area of the first stage compressor/impeller, by turning the propeller slowly in the normal direction of rotation (to avoid damage to carbon brushes in the starter/generator). Any evidence of damage, nicks, cracks, bent or missing blades should be brought to the attention of a qualified technician prior to starting the engine.

  • Propeller—rotate by hand

This is a valuable practice to develop a feel for characteristic engine sound and rotational resistance; it also helps establish a baseline of both feel and noise so that, should a change be detected on future prop rotations, appropriate maintenance investigation should be initiated. If an abnormal resistance is noted (for example, shaft bow; rotational freedom should be re-checked after about three minutes of additional cooling), hand rotation should be stopped at the point where the resistance is most obvious; representing 180-degrees displacement of the main rotating group, neutralizing the thermally caused imbalance as cooling continues. Rotational resistance is unusual except for the initial few hours of operation following replacement of the inter-stage air seals. Do not start the engine if the propeller is not free to rotate.

  • OAT sensor—check

Inspect for security and a clean, unpainted probe. Note: Engine performance and operating characteristics are a function of OAT and PA. The OAT sensor may pick up reflected ground heat and thus may read a higher ambient temperature than the OAT reported from an official meteorological observation source. The error will vary with sun position and type of ground surface.

  • Exhaust nozzle—check
  • Turbine blades: condition

If visible, check (a) exhaust pipe for concentricity; (b) condition of rear (third stage) turbine blades and eight EGT thermocouples (only with EGT system; ITT system temperature probes are not visible); (c) evidence of residual oil in the tail pipe. Turbine seal (oil) leaks must be noted for maintenance follow-up prior to next flight.

  • Aircraft orientation—into the wind

Strong tailwinds during a ground start can create excessive propeller loads against normal direction of rotation. Additionally, it causes back pressure in the tailpipe and may cause ingestion of exhaust gases. A headwind provides windmill power, ram inlet air, and a clear exhaust path. Except as noted in some AFMs and POHs, there are no wind restrictions for engine starts because maximum tailwind is a function of other start conditions, e.g., start-bus voltage, residual turbine temperature, and ambient conditions.

For additional information concerning TPE331 design and operation, please contact Rob Erlick at 480-399- 4007, or send an e-mail to [email protected]. Honeywell’s TPE331 Pilot Tips booklet is available on the Honeywell App or online at https://pilots.honeywell.com; register with name and email, point to “Engines” then “TPE331” then click on “Pilot Tips”.