This article originally appeared on the P&WC Airtime Blog.
If you’re flying a PT6A-powered aircraft and the propeller has unexpectedly locked into place, don’t panic: there’s most likely a simple explanation and an easy solution. Our expert explains all.
WHAT IS PROPELLER LOCK-UP?
Before starting a turboprop engine, it is common practice for ground personnel to manually turn the aircraft’s propeller to make sure it’s rotating freely with no abnormal noises. When moved slowly, the propeller will usually turn with ease.
But not always. Sometimes it doesn’t turn at all, or it rotates while making a scraping noise.
Most likely, this indicates a thermal lock-up, says Stéphan Michon, senior technical specialist at Customer First Centre, the response centre that operates around the clock to support customers worldwide.
Propeller lock-up is a known, easily resolved phenomenon that occasionally occurs with PT6A and some other engines, as a natural consequence of how these turboprop engines are designed. Stéphan Michon, senior technical specialist, Customer First Centre
There’s no sure-fire method of preventing propeller lock-up, although there are steps you can take to reduce the probability of it occurring, as discussed below.
The good news is that it can be handled with the cheapest, simplest solution of them all: doing nothing.
WHY LOCK-UP HAPPENS
Regardless of the aircraft’s mission type, propeller lock-up is most likely to occur when landing in a cold environment, where the outside air temperature (OAT) is low.
Fundamentally, it is caused by the temperature differential between the engine’s external casing and the inner components.
“When the engine is shut down, the cooling rate of the various components varies,” notes Stéphan. “Since the outside case is typically a relatively thin sheet of metal that’s exposed to the cooler nacelle temperature, its cooling rate is quite fast. On the other hand, the power turbine disks in the hotter engine interior have a slower cooling rate.”
The differing rates of thermal contraction combined with the extremely small tolerances within the engine sometimes causes temporary propeller lock-up.
WHEN LOCK-UP HAPPENS
The window of opportunity for thermal lock-up to occur is quite short, as Stéphan explains.
At some point, when the outside case has returned to its ‘cold’ state, but the disk assembly is still warm, the power turbine disk and blades’ outside diameter will still be slightly enlarged due to slower thermal contraction and will touch the shroud’s inside diameter when rotated. They may lock into place as a result of friction, causing propeller lock-up. Stéphan Michon, senior technical specialist, Customer First Centre
In other words, lock-up only happens during the relatively brief period when the engine’s internal components are still cooling down after the engine has been turned off.
As Stéphan is quick to point out, propeller lock-up does not mean there is something wrong with your engine. He compares it to the wipers on a car freezing to the windshield – something that happens occasionally in specific circumstances, but does not signal any underlying issues.
“Over time, propeller lock-up becomes less likely because normal wear on the turbine blades will slightly increase the tip clearances,” adds Stéphan. “It’s therefore slightly more frequent with brand-new engines or engines that have just been overhauled or refurbished, when the clearances are at their smallest.”
HOW TO DEAL WITH LOCK-UP
Propeller lock-up will resolve itself once the engine interior and exterior temperatures have somewhat equalized. That means all you need to do is wait for it to finish cooling down, with no corrective action required.
How long does this take? It varies depending on the aircraft, the engine model, OAT and other factors. There’s no way to put an exact number on it, says Stéphan, but it typically takes around five to 25 minutes for the engine interior to cool sufficiently.
If you don’t realize that the propeller has locked, you can still safely start the PT6A engine. Once it starts, the outside case will warm up quickly, eliminating the temperature differential and releasing the propeller lock-up.
While thermal lock-up is more likely to occur when landing in a cold environment, it will also be resolved more quickly, since the engine internals will cool down more quickly compared to a warmer environment. Conversely, in a warmer environment, there may be a longer delay before lock-up occurs after shutdown, and the propeller may remain locked somewhat longer as well.
Stéphan suggests a couple of tips for proactively reducing the likelihood of propeller lock-up when landing in a cold environment:
You can idle the engine for slightly longer – say 30 seconds to one minute – before you shut down the engine. You can also do one or two dry motoring cycles immediately after the shutdown, meaning that you turn the engine with the starter (within its limitations) while keeping the fuel and ignition off, which will draw cool air inside. Stéphan Michon, senior technical specialist, Customer First Centre
Both of these steps will help to cool the power turbines, making it less likely that thermal lock-up will occur due to a temperature differential.
If the situation is not resolved as described above, or if you have any doubts, consult the relevant section of the engine maintenance manual or contact Pratt & Whitney’s Customer First Centre for assistance.