TB20 engine fire, due to too much priming

But I wonder how much over-priming you would need to do for that to happen?

Normally, overpriming alone should not cause this case of fire. On all engine installations I have come across so far, there is a vent somewhere that will drain excess fuel out of the engine compartment before it can reach the air filter or other sensible/inflammable parts. In fact, “my way” of starting (or getting them started by students) cold fuel injected piston engines is to prime very generously, unless some precise figure is given like in the TB20 case. In winter, you often have only enough battery power for two or three starting attempts, so prime as if there is no tomorrow… On some piston twins like the Cessna 400 series, you prime a cold engine until you can see the fuel run out below the engine cowlings.

It is sometimes very difficult to determine whether an engine wouldn’t start because of insufficient or too much priming. In my experience, many pilots only know the “if it doesn’t start, prime more” and then flood the whole engine compartment with fuel. I’ve been there, too. It is probably very difficult to set a cold engine in cold weather on fire due to too much priming but when trying to start a hot engine and continuing to prime when it wouldn’t start, is a good way of achieving that.

The C172SP (fuel injected Lycoming IO-320) has some trouble with this. Therefore, the start procedure involves turning on the electric fuel pump until there is a fuel flow indication, then pull the mixture to idle, crank the engine and once it fires, push back in the mixture. The idea is that with the mixture on idle, a potential fire would be limited to the fuel that was injected.

Unfortunately there are a million “this is the ultimate way to start an aero engine” techniques taught be the pundits but none of them work with all engines. When I used to rent out my C172M, 8 out of 10 renters were unable to start it with a hot engine and I had to intervene when fuel was dripping out of the cowling…

I have seen an aircraft badly burned by over priming. When it is worse is on a cold day with a wet tarmac. The fuel sits on top of any standing water puddles and a back fire sets it off. As a result, if I over prime the engine, I move it so the fuel is not near the exhaust pipe. The intake manifold should have a bottom drain, but these can get plugged and are rarely checked to see if they are clear. This sets up a very dangerous situation if an over prime doesn’t drain on to the tarmac.

As a result, if I over prime the engine, I move it so the fuel is not near the exhaust pipe.

How do you do that with a Cessna 404 with two pilots and nine passengers on board? I have around 2000 hours on 4-series twin Cessnas and during that time have made quite a few puddles of fuel on the tarmac. Especially in cold weather, where you usually have a single attempt at starting the first engine on battery power. Not once has that fuel on the ground ignited.

Unfortunately there are a million “this is the ultimate way to start an aero engine” techniques taught be the pundits but none of them work with all engines.

This is correct. But as said, unless there is a specifc procedure for a specific engine, the “rather more fuel than necessary” way usually works quite well.

It is sometimes very difficult to determine whether an engine wouldn’t start because of insufficient or too much priming

Agreed. I have 8 or 9 years on a number of PA28’s Archer II’s and I still dont know if I have overprimed, My technique is to run the fuel pump during pre-start / walkaround checks and make sure there is some fuel being pumped into the engine. Then prime about 4 or 5 times (on the basis that the first few pumps arent actually putting any fuel into the engine – maybe I am wrong). Then just before starting I do a couple of pumps on the throttle.

This 9/10 works for me and the engine will start within 7 or 8 turns of the prop. Have I over or under-primed? No idea, though I wish I did. As people have said, there is no “one way” to start these old planes. In case of over priming / engine fire I dont put seatbelts on or close/latch the door until the engine is running. That seems a sensible thing to do anyhow…

One mistake I’ve seen quite a bit (and done myself!) is to push the primer in too quickly. You need to let the thing fill up with fuel before pushing. If done that way, I have never needed more than 2-3 ‘primes’ to start a C172 or C182. When hot, I usually don’t prime at all on either of the above.

That said, each installation has its quirks. I remember flying a C182 Peterson with an IO470 260 HP engine some years ago, and this needed a rather special procedure to get going.

I was hesitant to post this as it is rather embarrassing but I’m doing it anyway to warn others.

Tb20 is my first fuel injected engine so I didn’t fully appreciate the importance of not overpriming the engine.

Saturday was quite hot in London. I flew there to sit an exam, and whilst trying to hot start it, I went full rich, throttle half open, fuel pump on for a few seconds then mixture all the way back. Crank crank for a few seconds- nothing happened. Tried it for the second time. Wouldn’t start. After the third time I spotted some smoke coming from underneath the cowling. So I immediately stopped and investigated. Local engineer said I flooded the engine, had a quick look underneath and advised me to try hot start again. I did 2 hours later with 0 priming and engine started quickly!! Flew for an hour and it was uneventful.

I had this looked into properly today by my engineer. Cut a long story short, Turns out the air filter was burnt, but only on the outside. Inside the cowling all the wiring and pipes looked totally fine except near the outside where there was black stuff. I was very lucky.

Underneath the cowling everything investigated and no sign of damage beyond the air filter. Turns out for me this was a cheap lesson.

It could have been far worse so please be careful.

There are endless threads and resources about this so I will be educating myself properly. My first take away on hot start is to try to start it with no priming first. And never to prime more than once.

Best writeup on how to hot start an injected engine I have read is here:

https://vansairforce.net/threads/putting-the-adventure-in-airventure.115891/#post-905368

This is how I do it:

Consider the classic lever dance. It’s really a sweep moving toward lean from a condition of way too rich. First we flood it (full rich, boost pump on), which ensures we’re beginning the sweep on the too-rich end. Then we put the mixture at ICO and the throttle wide open; when we start cranking, no more fuel is added, but we’re supplying plenty of air. With each intake stroke the fuel/air ratio presented to the spark plug becomes progressively leaner and leaner. When the ratio passes through the stoich range (or close to it), spark makes fire. We close the throttle and open the fuel (the lever dance), hoping to remain in the sweet spot.

Rami1988 wrote:

I was hesitant to post this as it is rather embarrassing but I’m doing it anyway to warn others.

Thank you for posting this. By being embarrassed and not posting, nobody else has a chance to learn. None of us are born knowing everything (none of us die knowing everything either!). All we can hope to do is learn from others (and our own mistakes) hopefully before we need that knowledge!

So thanks for sharing!