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. This 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.
Back 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 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 taxi back to the ramp in hopes of finding some maintenance.
To make matters worse, the problem often appears intermittent, because by the time you taxi back, shut down and go look for help, everything has cooled off just enough to work properly. You find a mechanic, drag them over to your King Air to diagnose the situation, you fire up the problem engine and everything works just fine! OK, chalk it up to some odd glitch, load everyone back in the aircraft, taxi back out to the runway and once again no power on that engine! It’s a maddening situation on a boiling hot day.
Troubleshooting
The most expedient way to troubleshoot the oil-to-fuel heater is to run that engine for about 10 minutes; shut down and open up the R/H rear cowl door. You are going to check the temperature of the fuel bowl on the high pressure fuel pump. The HP fuel pump will be just forward of the FCU and its fuel bowl is above the oil-to-fuel heater (see photo, top right). BE CAREFUL! If the vernatherm in the oil-to-fuel heater is not working, this fuel bowl will be hotter than a poker! Use great caution in checking the fuel bowl or you will erase your fingerprints in the process. Warm is normal. If it’s too hot to touch, then your oil-to-fuel heater is the likely culprit.
Now what? Well, unfortunately for all of us, Pratt & Whitney does not allow the vernatherm to be changed in the field. The oil-to-fuel heater unit must be removed and exchanged. Once you find a suitable exchange unit and get it shipped to wherever you are, then it’s about a four-to-five-hour job for an experienced mechanic to change it out.
If you are in the boondocks, however, there is one other option. Keep in mind that this problem only happens on the ground, and that if you are able to get fuel streaming through the FCU, it will keep going. In other words, if you can keep your oil temperature from going too high, you’ll keep your fuel temperature in a viable range for proper FCU function. Once the FCU gets going, it won’t quit.
Inside the FCU is a labyrinth of diaphragms that cannot operate properly with super-heated fuel. The goal is to keep the fuel from becoming too hot before the engine is asked for full takeoff power. Once the FCU gets going, there is so much fuel going through the oil-to-fuel heater that the fuel cannot become super-heated.
So what happened to that King Air pilot with the urgent call? Here is what he did: He asked the tower for at least one minute after receiving clearance to take off. He taxied out on one engine only with his ice vanes stowed (up). After receiving clearance for takeoff, he started his “problem” engine. In doing so, he kept the oil temp down enough for the fuel temp to be acceptable to the FCU. The engine came up to full power and he was good to go.
He had suggested flying over to my shop but I told him he would be better off flying home and having his oil-to-fuel heater addressed there. Once safely home, he called me and thanked me profusely.
A Safe Work-Around
Before I go any further, let me stress here that I am all about safety. I do not advocate risky procedures or maverick bravado in the cockpit in any way, shape or form – period. That said, if you understand the systems that operate your King Air, there are certain instances where you can safely work around a problem until proper maintenance can be done. This is one of those instances, provided you diagnose it correctly.
First you must troubleshoot the fuel bowl on the HP fuel pump. Do not, under any circumstances, simply assume that you have an oil-to-fuel heater problem. You must check that fuel bowl after a 10-minute ground run. If it’s boiling hot, you’ve got a bad vernatherm in your oil-to-fuel heater. Next, you let everything sit and cool down. Your oil temp needs to get down to at least 40ºC.
Let the tower know that once you receive clearance for takeoff, you’ll need an additional minute to get going. Taxi out on your “good” engine only, with the ice vanes up (this helps keep your oil temp from red lining, which in turn keeps your fuel from super heating). Once you receive clearance, fire up the other engine. If you get all the proper indications, and you see that engine is now making power, it means you have ample fuel streaming through the oil-to-fuel heater and it won’t have time to super heat on its way to the FCU. Once airborne, the air going over the oil cooler will keep the oil temp down and the engine will operate normally.
Many of you who operate routinely in cooler climates could have a bad vernatherm in an oil-to-fuel heater and have no clue. It will never rear it’s ugly head until you get into really hot conditions on the ground.
When In Doubt, Don’t Go
A little knowledge can be a dangerous thing, and so it is with a measure of trepidation that I even address this topic of working around a bad vernatherm in an oil-to-fuel heater.
My mantra is “When in doubt, don’t go.” If you are unclear about how an FCU operates – don’t go. If you do not understand exactly what the oil-to-fuel heater does and why – stay put. If you cannot describe how the oil cooler works, please, stay on the ground.
Imagine that you encountered a no-power situation on the ground in hot weather, you felt certain it was correctly identified as an oil-to-fuel heater problem, but you still don’t feel comfortable with the work-around procedure; again, I say don’t go! Wait until early the next morning when the OAT is at its lowest. Check the problem engine to see if it fires up normally. If it does, then use your best judgment on how to proceed at that point. If you have any doubts, anywhere along the line, then don’t go. Do whatever it takes to get a knowledgeable mechanic on the aircraft to take care of things.
The good news is: It’s fixable. It’s a King Air. They’ve been around for a while.
Keep cool and fly safely.