This is what it looks like when the electric propeller heat is active on the ground with no propeller movement:
Burning rubber and 8 composite blades destroyed.
The electric heaters installed on most IFR rated aircraft propellers can crank out a ton of energy in order to keep up with the extreme cold found at altitude. The heat will melt off any accumulated ice and prevent further ice from forming. Ice on propellers is a bad thing for two reasons; it reduces the thrust efficiency of the propeller, and, it creates an imbalance that can cause propeller and engine damage. That same heat will do damage to the propeller if there isn’t sufficient cooling available.
Normal operations would have the propeller heat switch in the OFF positions at all times, except when you want the prop heat applied during icing conditions at altitude. Unfortunately that switch was left ON in this situation.
Now, there is also fail safe in the scenario to prevent this expensive repair from happening. Normally the prop heat will only operate when a pressure switch senses that you have oil pressure on the affiliated engine, hence a running engine, thus the prop is supposed to be moving and can provide some cooling to itself via the air it is slicing through. So there are two switches that must be activated for the prop heat to operate, one is automatic and senses oil pressure, and the other is human controlled.
So what went wrong here? Although this aircraft is fairly new to the fleet, there are many more like it within the fleet, so this is not a problem with personnel being unfamiliar with aircraft systems.
Further investigation found that the pressure switch had failed at some time in its past history. In order to make the system operational again, the pressure switch was bypassed or hot wired. This means that the system will now apply heat to the propellers without the engines running. The safety has been bypassed, which means its up to personnel to ensure the switch is always OFF. Not really a bad thing as long as you remember to turn it off….right?
This is what I like to call “being setup for failure”. No one intended to fry the props. However a safety was bypassed to keep the operation rolling. Stress happens and the fragile human brain forgets to fix that hot-wire job when the aircraft returns from its flight. Time passes and the aircraft is sold. New operator has no immediate knowledge of the potential problem and all systems appear to work. All your waiting for now is the right combination of forgetful human nature and WHOOF, you have two cooked props.
So what are the lessons that can be learned from this expensive and sad situation:
- Always follow the pilots check list – the check list is there to make sure your brain doesn’t forget any steps, like turning off things that have potential to do damage.
- Always follow the full maintenance procedure – A full blown functional check of the prop heat should have found this fault before it caused damage, never trust the guy who did the job before you.
- Fix the problem the first time – Procrastinating only causes more problems in the long run, the pressure switch is small and cheap when compared to the propeller it is protecting.
- Prepare for the worst – Always check that the cockpit switches and circuit breakers are in the normal or power down positions BEFORE you apply ground power to the aircraft.
At the end of the day, everyone is responsible for their actions. If each of us endeavor to do the job right the first time, we greatly reduce the risk of failure. Every time we take a short cut, or skip steps, we increase the likely hood that something will go wrong. Then all it takes is time…and eventually it will go wrong.
Does this sound familiar – “We never seem to have time to fix it properly right now, but we always have plenty of time to fool around with it over and over again for the next 3 weeks.”
Lets remember to “Setup our colleagues for success…not failure”