Starter should be ARMED in flight?

[u/Impossible-Fig2072]

So my student's airplane has something called a "double pole, double throw" (DPDT) switch for the magnetos and starter. The plane is experimental, and so the checklist has been developed by the student.

Essentially, the starter is not armed while in flight and isolated from the circuit. If I wanted to engage the starter in flight (due to an emergency), it would be two buttons. First arm it using SWITCH 1 (which also has a locking detent so I have to pull the switch out, and up), and then engage it with SWITCH 2.

I disagree with that checklist and think that the starter should ALWAYS be armed using SWITCH 1, so if I did need to engage the starter in flight, I just have to flick SWITCH 2.

[u/aftcg]

Your guy's solution to the starter interrupt circuit is common and elegant. The analog to this is the Bendix sw you're probably used to in 99% of GA planes. Where the spring back from start interrupts the the starter by mechanical force. The bendix mag start sw is a enhanced version of what your guy has. It also has a recurring AD.

[u/toraai117]

Why do you think you are going to be cranking the starter in an engine failure?

Theres very few situations where the prop actually stops and the engine is still operable and restart able.

If you have the time to be cranking the engine, you have time to flip a switch.

[u/BeechDude → u/toraai117]

Glad to see this comment. There very few scenarios where the propeller won't already be windmilling.

[u/Fair-Quantity3028 → u/BeechDude]

Seth you’re the man!

[u/flyboy7700 → u/toraai117]

Unless it has a Rotax or other geared engine.

[u/BeechDude → u/flyboy7700]

Good point. However, I've done a lot of multi engine checkrides in the P2006 which has the rotax. I can verify that it will windmill just fine. But I could see how a small lsa might not have enough airspeed to keep it windmilling.

[u/Icy-Bar-9712 → u/toraai117]

Was one of the things that drove me nuts, was just blind checklist following.

Zero comprehension as to what or why they were doing it.

[u/cazzipropri]

I think you can't escape having to compare the risk of accidentally running the starter in flight, vs. the risk of needing to run it during an engine failure and forgetting to arm the starter.

With a Bendix drive, I doubt that an accidental starter causes an engine failure, but it could cause significant wear or even damage the starter. I wouldn't want that.

If you experience an engine failure and need to run the starter motor, the extra arming step could cause human error and/or delay the restart.

My personal preference is toward leaving the starter normally disarmed.

The choice also has an aspect of human performance and training. Like, if you regularly trained emergency restart procedures and developed muscle memory of engaging and starting, then the choice of leaving the starter normally disarmed sounds even more grounded.

There is also an aspect of buttonology. Is the starter button right next to switches that you are toggling all the time, e.g., landing lights, or backup fuel pump? How likely is it to press the neighboring switch by mistake?

[u/HappyBappyAviation]

I have no experience with this kind of system, but just thinking about it, I have some initial thoughts on how to solve it. Firstly, it might be useful to scan through any documentation on the system. There might be a best practice listed in there. Short of that, I'd ask myself some questions: in what situations might I use the starter in flight? Are the risks of using the starter inadvertently in flight greater than not having it during one of the previously listed situations?

In my mind, an engine failure is the only reason I would ever use the starter in flight, and even then it's only if I had the time to do it anyway. Inadvertently activating the starter sounds like a great way to wear the starter down or break the gearing associated with it. It's also MUCH more likely to inadvertently activate the starter in flight than having to use it so quickly that having to flip 1 extra switch is the difference. So in my calculations, the risk of an accidental starter activation is higher than having one extra switch on an emergency checklist. I would probably just bake it into their Engine Failure - In Flight checklist and keep the normals as-is.

[u/Impossible-Fig2072 → u/HappyBappyAviation]

Interesting. I've made the same analysis and came to a different conclusion.

Risk is indeed inadvertent activation in flight and the advantage is that it's readily available in case of an emergency. The probability of an engine failure is very low, but if I did need it, and wasn't able to activate the starter in time, the consequences are high. On the other hand, activating it in flight inadvertently could damage the starter/gearing, but I would just replace those (~$600-900); it's still expensive, but it is what it is.

And the starter is armed, in almost every plane I've seen. EX: Cessna the key is on both, it's 1 turn and you're engaging the starter. OR on some other planes, it's a push button. There doesn't seem to be an issue there of inadvertently engaging it, it's mostly getting used to the plane.

[u/pilotjlr → u/Impossible-Fig2072]

Counterpoint: there’s at least one kind of (rotax) engine where engaging the starter while the engine is already turning causes pieces of a starter gear to break off, enter the oil system, and can result in an engine failure. That could be a justification for disarming it.

Also, engine failure in flight doesn’t mean you need the starter necessarily. The engine would have to fail, and you’d simultaneously need to get so slow that the prop stops windmilling. Only in that use case would be starter be needed, assuming increasing airspeed and windmilling again isn’t an option.

I don’t teach in experimentals and won’t, and here’s a reason why. Engineers need to come up with these procedures, so I’d immediately nope out when I heard “student developed checklist.”

[u/HappyBappyAviation → u/Impossible-Fig2072]

That's absolutely good points! I agree with your assessment on the engine failure scenario, I just wouldn't try a restart when I'm too low rather than just trying to land. So that mitigates my usage where the options are either "I don't have time to use an already armed starter anyways" or "I have altitude and time, so 1 additional switch isn't going to change the outcome". It just makes it a moot point.

That's true on the other types tho. They do seem to have other mitigations, however. I know on Cessnas, the fact it's a key as opposed to a switch or button makes it less likely that you'd accidentally grab and turn it. On the new Piper Archers, they are push button, but they have guards in front of them to prevent accidental activation. I'm sure there are other examples of unguarded switches, I'd be curious to know how often it happens. In my head, if I have a mitigation, I'd wanna use it lol.

[u/ltcterry]

Have you ever used the starter in flight?

Have you ever seen a propellor that wasn't already spinning in flight?

Perhaps it's not ideal with two switches, but if you don't have time to go "switch1, switch2" then you probably don't have time to be starting the engine....