In-Flight Incident: My Experience with a Throttle Malfunction | Pilot's Story

russ_watters · Jan 10, 2024

This was previously posted in the moderator's forum:

I'm not sure I mentioned it here, but I got a pilot's license a couple of years ago. A life goal of mine.

I did a 60 mile out-and-back flight on President's day Monday for time/experience building (requirement for IFR), solo. On the way back to my home airport I pulled the throttle back to descend to landing and....nothing happened. *Jiggle*. Still nothing. Still at 90% power.

I did a bunch of 360s to stay out of Philly airspace and kill time while figuring out what to do. I tried calling my flight school/instructor on the radio and cell, but they were closed (after 5pm) and my cell had no service at 5,500 ft (ASL; ground is about 500'). Circling enabled me to descend some without overspeeding the airplane.

I leveled-off at 2,500 ft and flew toward my home airport, still deciding what to do. At that point I knew I was going to have to shut off the engine to land, I just hadn't figured out the details. I decided I needed a longer runway: my home airport is 3,300 ft and 10 miles away is one that's 5,800 ft, so I diverted there. I called their traffic advisory freq and let them know I was coming in with a problem, then switched to Philly ATC and let them know. I didn't declare an emergency, I just wanted them listening in case something went wrong (throttle suddenly unstucks to idle?).

I did a wide, sweeping turn from the downwind* leg 3 miles from the airport. With no way to slow down or descend in level flight, I did a forward slip to bleed energy all the way through until I was on a low and fast final approach. About half mile from the runway at 107kts and 130', (about 50% faster and half as high as normal). I recovered from the slip, set the mixture to idle/cutoff to kill the engine and glided-in. Dumped flaps and landed pretty much normally 1,500 ft from the start of the runway. I rolled to a stop just short of a taxiway, but a couple of guys from their flight school were listening and came out with a tug to get me off the runway. Then I called the owner of the flight school I rented the plane from to tell him where I parked his plane.

Apparently a cotter pin broke and a nut worked itself loose on the throttle linkage. There was no way to fix it in flight. Also apparently this is a Thing that happens (though it's not in the POH/emergency procedures) - there's a couple of Youtube videos about it. Unfortunately I wasn't videoing it (which I often do) because I figured it was going to be a boring flight! There's some ATC recording of their end of the conversation though.

Then; Uber back to home airport to get my car, drive home, call girlfriend and mom, drink. Breathe.

*I did land the "wrong" way though. Winds were calm to variable and I got a bit of tunnel vision about what I was doing (landing, NOW). There were no airplanes in the pattern and I was mostly on Philly ATC frequency, not the airport traffic advisory frequency, so I didn't catch which runway(direction) they were using (though if I were paying attention I would have guessed correctly). Not illegal, just not particularly safe; oddly, planes in uncontrolled airports aren't even required radios or to communicate at all. Calling Philly ATC was a good idea though because they held a jet on the ground and another in the air while I landed and got the plane off the runway.

Filip Larsen · Jan 11, 2024

Good work keeping your cool and recovering almost to perfection given you had a non-functional throttle regulator. Just goes to prove that regular GA training in emergencies in general and engine failures in particular really are a must, including learning to keep your cool and being able to make an unpowered (gliding) approach and landing if everything else fails.

sbrothy · Jan 11, 2024

What a scary experience. as Filip said: cudos for keeping cool in that situation, landing direction be d*****. I'd expect such cool-headed handling would be worth something on your certificate, but as I've recently had verified, the universe doesn't work the way I think it does.

I'm getting more and more enlightened all the time thanks to this forum though.

Lnewqban · Jan 11, 2024

Could the engine be re-started with the throttle in that position?

Flyboy · Jan 11, 2024

Lnewqban said:

Could the engine be re-started with the throttle in that position?

Wouldn’t recommend it. Could damage the engine from the abrupt acceleration to 90% power.

Superb airmanship, Russ. You followed the emergency priority flow correctly: Aviate, Navigate, Communicate. It’s easy to critique your performance after the fact, but you brought the plane down safely, no one was hurt, and the plane was fixable. There’s definitely an embarrassed mechanic out there somewhere, but hey, it’s an opportunity for them to learn.

Lnewqban · Jan 11, 2024

Flyboy said:

Wouldn’t recommend it. Could damage the engine from the abrupt acceleration to 90% power.

I mean, is there any safety mechanism that protects the engine from starting up at high rpm's?

Also, it seems to be a very short distance between the handle and the carburetor, as to have a complex actuator that could fail and get disconnected.

Car carburetors have a return spring that prevents situations like this.
Just trying to learn about the failure, if you know the details of this throttle mechanism.

russ_watters · Jan 11, 2024

Lnewqban said:

Could the engine be re-started with the throttle in that position?

Flyboy said:

Wouldn’t recommend it. Could damage the engine from the abrupt acceleration to 90% power.

Lnewqban said:

I mean, is there any safety mechanism that protects the engine from starting up at high rpm's?

You definitely can, but I don't know about "would not recommend".

There was some discussion in the flight school afterwards about trying to shoehorn my situation into one of two commonly practiced engine failure situations. Low altitude, you practice a failure abeam your touchdown point in the downwind leg of the pattern: just make a 180 degree, sharper than average turn and land. The other is a high altitude failure where you do a series of 360s just off the end of the runway to lose altitude then set yourself up to land from a similar point as the first one. I considered these at the time and decided they were unnecessary tests of my skills and the engine. Partly because the flight school sold the Cessna I was learning in right before my checkride and I probably only had 20-30 hours in the Piper I was flying -- and Pipers don't glide as well (and worse still with a windmilling propeller vs one at idle). But also, the engine:

You shut the engine off by pulling the mixture to idle/cutoff, shutting off the fuel. The spark plugs are still firing and the propeller is windmilling. In theory if you put the mixture back up it'll roar back to life. In theory. I considered that and decided I wasn't anxious to test it.

Not knowing the exact nature of the failure, I didn't know if the throttle would rattle itself into a different, less optimal position when restarting. Note that cruise power was probably the "best" position for the failure. If the linkage had broken during climb-out I would have been stuck at full throttle and would have had no choice but to shut the engine off to lose altitude - I wouldn't have even been able to maintain constant altitude without overspeeding. If it had broken during my descent it would have been much worse because I would have tried to level off at 1000' above the ground and found I was unable to stop my descent, and maybe not near enough to the airport to get there.

Or just - maybe if I'd shut it off early I'd fail a test of nerves and forget what to do when I see the trees rushing-up at me. I figured it was safer/easier to just shut it off when I was absolutely certain I'd make the runway without it.

They do tell you to bring the throttle up "smoothly" in normal circumstances. If you do it too fast the engine can cough/sputter a bit. But I don't know about damage and nobody mentioned it. One guy I saw on Youtube afterwards was using the mixture to choke-off the engine and had pretty decent control of the RPMs that way. I don't know how far I'd considered that - you of course adjust the mixture for efficiency normally, which affects RPM. But I didn't know you could go that "deep" and, again, I wasn't anxious to learn or test the throttle stickiness at the time.

Lnewqban · Jan 11, 2024

Thank you, Russ.

View attachment 338434

dlgoff · Jan 11, 2024

Lnewqban said:

Thank you, Russ.

View attachment 338434

The attachment doesn't come up for me. Goes into a black hole/ page not found.

Flyboy · Jan 11, 2024

Lnewqban said:

I mean, is there any safety mechanism that protects the engine from starting up at high rpm's?

Also, it seems to be a very short distance between the handle and the carburetor, as to have a complex actuator that could fail and get disconnected.

Car carburetors have a return spring that prevents situations like this.
Just trying to learn about the failure, if you know the details of this throttle mechanism.

There is not. The kind of failure that occurred is extremely unusual and probably happens once a year or so across the entire US GA fleet.

The throttle is generally a direct cable linkage to a push/pull knob in the cockpit, with a lever linkage on the carburetor itself driving both the butterfly throttle plate and fuel flow adjustment. Without seeing the particular unit in question, I won’t speculate on what else is going on mechanically.

A return spring is just another thing to break or malfunction. It’s better to fail and stay put, imo. Gives you more time to figure out what’s going wrong.

russ_watters said:

You definitely can, but I don't know about "would not recommend".

There was some discussion in the flight school afterwards about trying to shoehorn my situation into one of two commonly practiced engine failure situations. Low altitude, you practice a failure abeam your touchdown point in the downwind leg of the pattern: just make a 180 degree, sharper than average turn and land. The other is a high altitude failure where you do a series of 360s just off the end of the runway to lose altitude then set yourself up to land from a similar point as the first one. I considered these at the time and decided they were unnecessary tests of my skills and the engine. Partly because the flight school sold the Cessna I was learning in right before my checkride and I probably only had 20-30 hours in the Piper I was flying -- and Pipers don't glide as well (and worse still with a windmilling propeller vs one at idle). But also, the engine:

You shut the engine off by pulling the mixture to idle/cutoff, shutting off the fuel. The spark plugs are still firing and the propeller is windmilling. In theory if you put the mixture back up it'll roar back to life. In theory. I considered that and decided I wasn't anxious to test it.

Not knowing the exact nature of the failure, I didn't know if the throttle would rattle itself into a different, less optimal position when restarting. Note that cruise power was probably the "best" position for the failure. If the linkage had broken during climb-out I would have been stuck at full throttle and would have had no choice but to shut the engine off to lose altitude - I wouldn't have even been able to maintain constant altitude without overspeeding. If it had broken during my descent it would have been much worse because I would have tried to level off at 1000' above the ground and found I was unable to stop my descent, and maybe not near enough to the airport to get there.

Or just - maybe if I'd shut it off early I'd fail a test of nerves and forget what to do when I see the trees rushing-up at me. I figured it was safer/easier to just shut it off when I was absolutely certain I'd make the runway without it.

They do tell you to bring the throttle up "smoothly" in normal circumstances. If you do it too fast the engine can cough/sputter a bit. But I don't know about damage and nobody mentioned it. One guy I saw on Youtube afterwards was using the mixture to choke-off the engine and had pretty decent control of the RPMs that way. I don't know how far I'd considered that - you of course adjust the mixture for efficiency normally, which affects RPM. But I didn't know you could go that "deep" and, again, I wasn't anxious to learn or test the throttle stickiness at the time.

My concern is more about the volume of fuel that would be getting delivered to the cylinders while you’re turning over the engine, potentially flooding the engine. That’s another issue that you really don’t want to have to juggle while in glider mode. A panicked pilot might keep cranking on the starter and burn it out trying to start the engine.

I make a habit of not second-guessing pilots who handle the emergency well and make it home in one piece. I’m not in the cockpit, I don’t have the whole picture. But I think you handled the situation perfectly, especially with your clarification of your thought process and decision making.

sbrothy · Jan 11, 2024

When I worked for Maersk Data, they held a New Years party where they invited notable/celebrity persons. Among them was Stefan G. Rasmussen who relayed his experience landing the SAS Flight 751. That was quite awesome hearing that first hand.

B.S was another invitee but still, the Captain's story is what I took away from there.