Page 14 - Volume 12 Number 8

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.
12 • KING AIR MAGAZINE
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. KA
Dean Benedict is a certified A&P, AI with over 40
years’ experience in King Air maintenance. He’s the founder and former owner of Honest Air Inc., a “King Air maintenance boutique” (with some Dukes and Barons on the side). In his new venture, BeechMedic LLC, Dean consults with King Air owners and operators on all things King Air related: maintenance, troubleshooting, pre-buys, etc. He can be reached at dr.dean@ beechmedic.com or (702) 773-1800.
AUGUST 2018