An older twin engine turboprop was known for posting false overheat warnings that meant either there was a wheel well fire or a serious (450F) overheating in an air duct in the leading edge of the wing. Both are very serious conditions requiring immediate action. In the air this results in exercising an emergency procedure and an immediate landing. If it happened on the ground, the flight would be cancelled and the passengers hastily removed.
There had been accidents in the past where these conditions really existed and the aircraft were lost. So, it is not something one could take lightly even if the crews expected it was just another false warning.
False Warnings. Crews can become complacent, thinking it is just another false warning. False warnings are an insidious danger because they reduce the crew’s performance in an aerial emergency.
Ground crews get frustrated chasing the false warnings.
Management sees the rapidly escalating costs as flights return to the departure location, or land at the nearest airport, or simply do not depart. Of course, this ripples through the entire schedule. And there are overtime hours for staff trying to return the aircraft to service.
Passengers get scared and reluctant to fly with the airline. And word of mouth exacerbates the situation.
What this operator knew was that the false warnings happen most often in damp conditions. And the maintenance crews knew that false warnings can often be cleared on the ground by doing a long run-up. This seemed to drive the damp out and “fix” the problem. But it wasn’t a very satisfactory solution. And it did not solve the problem.
The aircraft manufacturer suggested various waterproofing schemes around the temperature sensors themselves, but that never really worked very well. Sooner or later the damp would creep back in and they would be worse off. Worse, because it was a lot harder to drive the damp back out with all that sealant around it.
The whole situation came to a boil. The crews were upset and a complaint was filed under the Safety Management System which required prompt resolution of the problem.
The operator called us. As is always the case, our understanding of aging aircraft and electronics helped us troubleshoot the system.
We tested and redesigned the circuit.
First, we examined the circuit itself. This was an old electronic circuit design lacking all the normal design features that one would expect from a modern design to isolate and protect the circuits from environmental effects. It was also a very high impedance design, which means just that it would take very little to set it off—like a little bit of dampness. To demonstrate that, we used the old “wet finger test”. Any electronics engineer or technician will remember this. We placed a slightly dampened finger on the terminals of the temperature sensor and, sure enough, the red overheat warning was flashing away in the cockpit.
Then we redesigned the circuit. We created a robust low impedance design that was inserted between the sensor and the circuit card that isolated the sensitive circuit card from dampness on the switch. Plus, we made it easy to field fit.
That was that. No more false warnings. No more lost flights. No more scared passengers. And no more money down the drain.