You’ve just landed and shut down your TPE331-powered Twin Commander. It’s a hot summer day, and you need to refuel and take off again in about 20 minutes. Given the conditions, is there anything special that needs to be done on the next engine start? The short answer is yes.
First, as soon as the passengers deplane and are well clear of the aircraft, pull the props through at least 10 revolutions. This is to quickly get as much cooling air through the engines as possible to avoid a condition known as shaft bow. This occurs when the underside of the engine cools faster than the top side, where the accessories are mounted. This uneven cooling actually causes a slight bending of the main shaft, and may cause the rotating group components to come into contact with their shrouds.
This contact can be a light touch, or in extreme cases can cause such a binding that you might be able to perform pull-ups on the props without turning them, until everything thoroughly cools and equalizes and the props turn freely again.
Pulling the props through also helps to cool the fuel nozzles, which helps to lessen the potential for coking of the fuel nozzles.
Once the aircraft has been refueled you need to perform a simple check to ensure there is no shaft bow. With two fingers placed about halfway down the leading edge of a prop blade, see if you can pull the prop through several inches – at least one blade width. That’s enough to turn the main shaft at least 180 degrees of rotation. If you feel no binding or hear no scraping sounds, you are cleared for the next step.
If, however, you feel any binding or hear scraping noises, stop immediately to avoid damaging the compressor shrouds. Ignoring this check could result in the engines complaining when started, sometimes loudly, with a resulting loss of efficiency. Let’s see, loss of efficiency means more fuel required to generate a given torque value, and a resulting higher engine operating temperature and operating cost.
Ok, you’ve checked the engines and found no shaft bow. What next? Remember what you were taught in ground school about maximum engine temperature prior to starting: ITT max indicated 300 degrees, and EGT max 200 degrees. If the temp gauge shows higher than that prior to start, there are three ways to get the temperature down below the max limitation:
- Wait until the engine temperatures drop of their own accord.
- Pull the engines through rapidly at least 10 turns, or
- Use the starters to motor the engines.
To motor an engine, depress the ignition switch to the MOTOR position and hold it there while you engage the starter. Let the engine spool up to about 15 percent rpm, then rotate the start switch to the OFF position. DO NOT release the ignition switch until the prop has stopped.
Why? Releasing the ignition switch as soon as ITT or EGT temperature drops to or just below the pre-start limit can cause what we refer to as a “booming” start, sending a nice shock wave through the engine and causing internal damage. It may not be catastrophic the first time it happens, but some level of damage occurs. This shock wave can and does travel up the P3 sense line to the fuel control and does wonders for the life of the bellows in the fuel control (Woodward). The Bendix FCU design is different, but the shockwave doesn’t help it either.
Also, there have been instances where the speed switch calibration drifts. If the engine is turning just fast enough that the speed switch circuit thinks the engine is above 10 percent rpm, it could dump fuel into the combustor and fire the igniters, just when you don’t want it to.
Once the temperatures are below the pre-start limits, go ahead and start. Do you remember the ground start modes to use depending on OAT? See the chart and explanation below.
ENGINE START MODES
The hotter it is outside the less temperature margin we have for starting, so above 32°C we need to get as much cooling air as possible through the engine during start to help keep temperatures within limits. Use series mode.
From 0 to 32°C we can use the serial start mode, parallel start mode, or use a ground power cart for assistance.
From 0 to -7°C use series start mode to account for battery energy loss due to cold temps.
Below -7°C we need to use a ground power cart to overcome the drag of cold oil and further loss of battery energy.
Happy flying
Mike Grabbe is a Technical Advisor with Eagle Creek Aviation Services. He has extensive experience on Twin Commanders.
