Maintenance Tip: Summer Heat and Engine Problems (Reprise)

Maintenance Tip: Summer Heat and Engine Problems (Reprise)

Recently, I’ve heard from several King Air owners who read this magazine and called me about my articles. All of these guys are fairly new to the world of King Airs and asked if some of my earlier articles could be run again. Even some longtime readers of this magazine requested a “refresh” on some of my old articles. What follows is the article from the July-August issue of 2012. It addresses an engine problem that can crop up when the OAT is scorching hot.

Hot weather conditions happen every day somewhere in the world, but right about now in the desert Southwest of the United States, we are feeling it big time. I received my first heat-related panic call back in May from a King Air pilot. He was at an airport near me, trying to take off and return home. One engine would not accelerate when he took the runway. He said he fire-walled the power lever but nothing happened, and to add to his concern, the idle was starting to decrease. Not a good sign. He returned to the ramp and conferred with a couple mechanics from a local tour operator who had suspicions about his fuel control unit (FCU), but suggested he give me a call.

The author pointing to the fuel bowl of the high-pressure fuel pump; the oil-to-fuel heater can be seen below it, behind a wire bundle. The most expedient way to troubleshoot the oil-to-fuel heater is to check the temperature of the fuel bowl after the engine has run for 10 minutes.

As soon as I heard him recount his steps and symptoms, I knew exactly what was wrong. It happens every summer around here. The problem is with the oil-to-fuel heater. Yes, you read that right, the oil-to-fuel heater and no, I haven’t lost my marbles. Trust me, I have seen this happen many, many times.

Most people would immediately suspect a problem with the FCU, or maybe a P3 problem (bleed air going into the FCU) or possibly a Py problem (air going from the FCU to the prop governor). But before I “accuse” the FCU of any wrongdoing, I want to go upstream to the oil- to-fuel heater and look around.

The oil-to-fuel heater has a vernatherm inside which should shut off the oil-to-fuel heater during extreme heat conditions. If this vernatherm is bad, then the oil-to-fuel heater doesn’t get the message to shut off, so it continues to heat the fuel as if the aircraft was at altitude and not 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 it. The FCU cannot function and the engine will 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 140º F, so your oil temperature is already up there. (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, but this also opens the rear bypass door and prevents airflow across the oil-cooler; now your oil temp climbs 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 could appear intermittent because in the time it took to find some assistance, everything has cooled off just enough to work properly. As soon as you get a mechanic there to diagnose the situation, you fire up the problem engine and everything works just fine! OK, you think it was just some glitch so you load everyone back in the aircraft, taxi back out to the runway and again, no power on that engine.

If the vernatherm inside the oil-to-fuel heater is bad, it continues to heat the fuel as if the aircraft was at altitude and not on the ground with a triple digit OAT. This doesn’t allow the FCU to function and the engine will not make power.


The most expedient way to troubleshoot the oil-to-fuel heater is to run the 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 first photo above). 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 finger prints in the process. Warm is normal, but too hot to touch is the sign that your oil-to-fuel heater is the 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. First you have to 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.

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 and the FCU will kick into action. Once the FCU gets going, it won’t quit.

The labyrinth of diaphragms inside the FCU 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, then the FCU will do its job and keep going. Once the FCU is operating, there is so much fuel going through the oil-to-fuel heater that there is no time for the fuel to become super-heated.

So, what about that King Air the pilot called about? 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 (this was back when I had one), but I told him he would be better off flying home and having his oil-to-fuel heater addressed there. He did so. Safely home, he called me and thanked me profusely.

A Safe Work-Around

Before I go any further, let me stress that I am all about safety. I do not advocate risky procedures or maverick bravado in the cockpit in any way, shape or form. However, 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 carefully check that fuel bowl after a 10-minute ground run. If it’s boiling hot, then let everything sit to cool down. Your oil temp needs to get down to at least 40º C.

Let the tower know that you’ll need about a minute to get going after receiving takeoff clearance. Taxi out on one engine 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 you are airborne, the cooler 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 its ugly head until you get into really hot conditions on the ground.

Don’t be Reckless

I cannot emphasize strongly enough that I do not advocate reckless operation of any kind. 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. I strongly suggest that you continue learning about your King Air every day.

When in doubt, don’t go. This is my mantra. If I was unclear about how an FCU operates, I wouldn’t go. If I didn’t understand what the oil-to-fuel heater does and why, or if I couldn’t say how the oil cooler works, then I would stay on the ground.

If you run into a no-power situation on the ground in hot weather and you feel it has been correctly identified as an oil-to-fuel heater problem but you are still not comfortable with the work-around procedure, then wait until early the next morning when the OAT is at its lowest. See if that engine fires up normally. If it does, then use your best judgement on how to proceed. If you have any doubts – don’t go. Do whatever it takes to get a knowledgeable mechanic on the aircraft to take care of things.

Keep your cool and fly safely.

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