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MAINTENANCE TIP
Summer Heat and Engine Problems
by Dean Benedict
Recently, hot weather has been a top news story all over the United States. Areas that don’t normally see triple-digit temperatures have been getting a taste of what we deal with every summer in the desert Southwest. TBhis got me thinking about an engine problem that can crop up when the OAT is scorching hot. And with all this talk about heat waves and high temperatures, it seemed like a good idea to revisit this situation.
ack when I had my shop, I got an urgent call from a King Air pilot at an airport not far from me. It was a very hot day with triple-digit temperatures. He was trying to take off and return home, but one of his engines would not accelerate when he took the runway. He said he “fire-walled the power lever” but nothing happened. Adding to his concern was the idle starting to decrease. He definitely had a problem.
He returned to the ramp and conferred with a couple mechanics from a local tour operator. They had suspicions about his fuel control unit (FCU), but they suggested he give me a call.
As soon as he told me what he had observed, and the steps he had taken, I knew exactly what was wrong. It happens every summer around here, and it’s not the FCU; rather, it’s a problem with the oil-to-fuel heater that’s throwing the FCU off. Yes, you read that right, it’s the oil-to-fuel heater. Trust me – I have seen this happen many times.
When an engine won’t accelerate, most people would immediately suspect a problem with the FCU; they might also suspect a P3 problem (bleed air going into the FCU) or possibly a Py problem (air going from the FCU to the prop governor). But experience has taught me not to dig into the FCU without first going “upstream” to the oil- to-fuel heater and look there.
Inside the oil-to-fuel heater is a vernatherm which shuts the oil-to-fuel heater off during extreme heat conditions. If this vernatherm goes bad, then the oil-to-fuel heater
20 • KING AIR MAGAZINE
doesn’t get the message to shut off. It continues to heat the fuel as if the aircraft is at altitude instead of on the ground with a triple-digit OAT. Super-heated fuel lacks the correct viscosity, and the FCU does not know what to do with this hot fuel. The FCU cannot function properly, therefore the engine does not make power.
There are a number of factors that contribute to this scenario: (A) You are on the ground, it’s really hot outside and the heat radiating off the ramp can be 140oF, so your oil temperature is already on the high side. (B) If you are in a King Air 200 with 4-blade props, you no doubt have your ice vanes deployed (down) to protect against FOD; however, this also opens the rear bypass door and prevents airflow across the oil-cooler, causing your oil temp to climb even higher. (C) You taxi out, maybe you sit in line waiting to take off, and if the vernatherm in your oil-to-fuel heater is not working, the fuel becomes super-heated and the engine won’t make power. So, you
AUGUST 2024