I am a new member to ICS, so this is my first post.

We had an incident where our 1961 250 Comanche was flying for about 10min and the pilot turned off the fuel pump. The engine sputtered and died. The pilot turned the fuel pump back on and the engine came back to life. After another 10 min the pilot turned off the fuel pump and the plane ran fine for the next 5-1/2 hours.

I would think that during run-up, take off and climb any air in the line would have dissipated. The only event that was different was that we drained the strainer the night before.

Does anyone have any suggestions on what might have caused this?

Thanks
David Begin
#17939

David,

Welcome to the Forum.

While I fly a twin, I have seen posts on this issue before but don't recall what to do about it - hope other 250 owners will post.

David,

What you experienced was a vapor lock of the engine driven fuel pump. People have died because of this quirk of the carbed Comanche 250's.

The engine driven fuel pump is cooled by the fuel going through it. The electric pumps are stronger than the engine driven pump. Therefore the engine driven pump has no cooling fuel flow through it when the electric pumps are operating. Add a down wind taxi prior to takeoff on a hot day with the electric pump on, and you get minimal cooling, boiling fuel in the engine driven pump, and a potential disaster with nothing to tell you that you are about to have engine failure twenty seconds after you turn off the electric pumps.

The late guru of all Comanche gurus, Maurice Taylor, developed the procedure below to counter this problem.

NOTE: This procedure applies to Comanche 250's with a carb. I think the procedures for fuel injected engines are different, but I am not familiar with them.

1. Before engine start, turn on the electric pump. Note increase in fuel pressure. Listen to the pumps work (you can't hear them after engine start). If the sound is like a Latin music beat (as the pumps slip into and out of phase), turn off the electric fuel pumps AND LEAVE THEM OFF UNTIL TURNING ONTO THE RUNWAY FOR TAKE OFF. If the sound is a steady beat, one of the two pumps is not working. Get it fixed before further flight. (If one of the electric pumps is not working and the engine driven pumps fails, a single electric pump will not deliver enough fuel for full power. It probably will permit the engine to run at reduced power, but pulling the throttle back with an engine failure at 100 feet is not exactly the first thing that comes to mind.) By the way, the tempo of the beat will slow down as the fuel pressure builds.

2. Start the engine with the electric pumps off. If the engine will not start without the electric pumps on, do you really want to fly with a broken engine driven pump? Keep the electric pumps off. This keeps fuel flowing through the engine driven pump, thus cooling it and keeping the vapor lock possibility to a minimum. And, if a vapor lock does happen, you will know it when the engine dies as you are taxiing.

3. Keep the electric pumps off until pulling onto the runway just prior to take off. (I know that is a repeat. It really is that important.)

4. Turn on the electric pumps just before turning onto the runway.

5. After take off, do not turn off the electric pumps until reaching a safe altitude. (I use 1500 feet).

6. After turning off the electric pumps, monitor the fuel pressure. If it drops, turn the electric pumps back. After 30 seconds, turn the electric pumps off again. Monitor the fuel pressure. Repeat as necessary. Each time the pumps go off and on, a little cool fuel goes through the engine driven pump, cooling it. Usually only two of three repeats are necessary.

The problem does not happen often. It has happened once to me (before I heard of this procedure).

It also happened to a very nice couple who were not so lucky: two fatal, two serious. And that crash is why chest straps are available for Comanches.

Hank
Henry A. Spellman
N5903P
1959 PA-24-250

Hank Spellman's explanation is absolutely correct.

I made the same heart-stopping discovery in my 1959 - 250, climbing out of White Plains, NY one hot, muggy, hazy day back in 1988 with my family on board and all those gazillion-dollar houses occupied by litigious owners below.

Maurice Taylor later gave me the same explanation re: vapor lock. The experience got me to thinking that there must be a better way to ensure the redundancy of the two electric auxiliary fuel pumps.

With this in mind, I modified the wiring to the two pumps so that each are controlled by a separate switch. I found that a split type Cessna Master switch worked perfectly.

My modification is described in some detail in the "TIPS" section of this website, under "03 - Fuel System", on page 27/40. As far as I am concerned, everyone flying a carbureted 250 should perform this mod. Simple and cheap.

If any questions, feel free to ping me at c-flhv@magtech.ca

Don Ostergard, ICS 3263
Drumheller, Alberta, Canada

Henry:

Thank you so much for the reponse. I have never heard of this issue before and I am glad you shared it with me and the group.

Thanks again

David Begin

David, My 1960 comanche 250 has a cooling shroud around the fuel pump, with a hose from the back of the engine baffles, to help cool the pump. I don't know if this is standard on all 250's or not. You might check. I've flown in 105 degs. and higher in Death Valley and never had a problem using the above procedure. IMO the above spelled out procedure should be placarded next to the pump switch and added to the POH. Ray B

Thanks Don - we will review the mod.

David Begin

Ray - we will check the plane to see if we have a shroud.

To all,

Hank is very correct. This problem also affects the Comanche 180 (engine driven fuel pump vaporization with the electric fuel pump on) although the 180 only has one electric fuel pump. I have experienced it only once but was very glad to be watching the fuel pressure after switching off the electric pump and knowing to turn on the electric pump as soon as I saw the fluctuation in fuel pressure. That particular time it took about 5 cycles to get the engine driven pump back up and running. I recommend that any time the electric fuel pump is turned off, that you leave your hand on the switch and count to 5 and monitor the fuel pressure guage. Any loss of fuel pressure and flip the switch back on immediately. I hope this is taught in the CPPP course. It is a vital procedure. Like Hank says, people have died from this.

There was a fatal accident involving Len Saxlid back in about 1994. He was from the North East and had been a Tribe Chief there. It happened in Sheridan, Wyoming. The area is elevation about 5000 feet and it was summertime hot. The landscape is rolling hills. There were 4 persons in the plane and an engine failure occurred not long after takeoff. Not knowing what to do, Len was forced to make an off airport landing into the side of a rolling hill. The front seat occupants died and the rear seat occupants lived. That accident initiated the Comanche society to start a shoulder harness program and many Comanches have shoulder harnesses today because of it. Many more Comanches don't have but still should have shoulder harnesses at least in the front two seats. Maurice Taylor helped with identifying why the aircraft lost the engine (there was no mechanical reason for it to quit) and spent much time and effort trying to educate the Comanche population about this phenomena. Some still remember and others unfortunately will have to learn the hard way. Hopefully this thread will reach as many Comanche pilots and owners as possible.

Sincerely,
Skip Dykema

David,

This is one type of the info in the CPPP course. Highly recommend it - We've taken it twice and may do it again this fall. http://www.comancheflyer.com/NS/cff_files/cf_ppp.htm