Aborted takeoff, engine not producing full power

Henning said:
Exactly! It's a performance gauge. If you aren't off the ground by the 1000' marker in a 172, unless you have major DA issues, you know you have a performance problem akin to an accurate 2000rpm indication. It doesn't really matter to us the detail of scope, it's a rough cross check for early determination of a problem is all, but it's easy and effective as one.
Click to expand...

Even if you do have high DA issues, it's a good idea to identify an abort point at every airport. As in, "I should be airborne or at a given airspeed by X marker, or I abort." 50 feet is best, and usually works, but occasionally short fields push you to other measures. It is possible to get airborne but not get out of ground effect near (but a little above) the aircraft's performance limits. Here, "short" means less than twice the 50 foot length given in the POH.

At pretty much any sea level airport with touchdown zone markers, I expect to be airborne in a 172 before reaching them. That's 1000 feet from the threshold.
 
MAKG1 said:
Even if you do have high DA issues, it's a good idea to identify an abort point at every airport. As in, "I should be airborne or at a given airspeed by X marker, or I abort." 50 feet is best, and usually works, but occasionally short fields push you to other measures. It is possible to get airborne but not get out of ground effect near (but a little above) the aircraft's performance limits. Here, "short" means less than twice the 50 foot length given in the POH.

At pretty much any sea level airport with touchdown zone markers, I expect to be airborne in a 172 before reaching them. That's 1000 feet from the threshold.
Click to expand...

Exactly, it's a data point you can use in multiple decisions. If a 172 was actually only spinning up 2000 rpm, you would be on the ground a lot closer to the marks than you are used to.
 
Johann said:
Is the performance I was getting consistent with running on one magnet?
Would it hurt if I did a static full power run up on one magnet next time I go just to see the RPMs I get?
Click to expand...

I'm not sure what you should expect to see at full power with one mag. I don't think it'd hurt to do a short check next time you fly, just don't run on one mag for an extended period.

Is there an engine monitor in the plane?
 
flyingcheesehead said:
I'm not sure what you should expect to see at full power with one mag. I don't think it'd hurt to do a short check next time you fly, just don't run on one mag for an extended period.

Is there an engine monitor in the plane?
Click to expand...
No, no monitor.
 
Henning said:
Your evidence?
Click to expand...
8000 hours of flying with such engines, and not once seeing that happen. Of course, you can't prove a negative. Bur perhaps you can arrange a demonstration where cold weather causes a mechanical tach to read 300 RPM low purely due to the temperature.
 
Ron Levy said:
8000 hours of flying with such engines, and not once seeing that happen. Of course, you can't prove a negative. Bur perhaps you can arrange a demonstration where cold weather causes a mechanical tach to read 300 RPM low purely due to the temperature.
Click to expand...

So nothing technical or mechanical in the way of evidence of rates of spring contraction by temperature?
 
Henning said:
So nothing technical or mechanical in the way of evidence of rates of spring contraction by temperature?
Click to expand...
You show me your data on temperature inside a tach when the engine has been running long enough to get the oil temp up in the green, along with your calculations, and I'll consider it. But empirical evidence would be a lot more convincing given that I've never seen anything like that happen in 45 winters of flying.
 
I had the speedometer in my '48 dodge show a much slower speed when I first got it out after years of storage. The lubricating oil had turned into a wax-like grease. After cleaning and relubing it showed an accurate speed. I wonder if the lube in this plane had gotten hard when it got cold which prevented it from reading a correct speed. When someone tried it later it was warm enough to thin out and show the correct engine speed.
 
I like these threads. The give me things to try out next time I'm in the 172. Really note what the RPM is static and on the roll. And see what the effect on RPM is with a single mag. I firmly believe in seeing what problems look like in a controlled setting first. That way an actual failure or emergency isn't my first time experiencing and reacting to symptoms. After the recent thread about electrical / radio failures, I had the tower give me the light gun signals so I could see them. It was very helpful.

I've tried to takeoff without pushing the mixture in while being a dumbass. Fortunately I lean aggressively, so applying full power just made the engine stumble and almost stop.

Is it even possible to lean the mixture back enough to get a 300rpm drop without it just crapping out? It certainly won't on a carburated engine.
 
I'm not buying 300 RPM on a single mag. The mag test fails at 150 RPM, and is usually less than 100 RPM, at 1700 or 2000. The most I've ever seen is just over 200, and that was with a broken spark plug lead.

It sounds very much like a restricted intake of one sort or another.

Is there an intake duct that can collapse under load? I've seen that happen in some cars, particularly when the air filter needed replacing 10,000 miles ago.

Has anyone considered the possibility of restricted exhaust? Like, a critter nest in there or a disintegrating muffler?

One thing that could do that is badly retarded timing, but I'd sure hope the mechanic checked that. It would also make the engine difficult to start, but cold weather can do that all on its own.
 
Last edited:
I would think FOD obstructing the intake or exhaust, or a deformity, would not vanish when the mechanic went to look at it. Those could all cause the symptoms, but the symptoms would likely remain. I like the sounds of ice on the air filter. It's about the only thing that could vanish without a trace while nobody is looking. And if it is clear or internal, one could easily not notice it on the pre-flight.

I too have a hard time seeing a single mag result in a loss of 300 RPM, let alone still run smoothly with that kind of failure.
 
Pedals, please post your results if you get to it before I do (I won't be flying until next week at least).

MAKG, the run up for this airplane is at 1800 RPM, and as you say it fails at 150 RPM drop. The drop was less than that in my run up, so I see your point. I would still want to try it though, I can see myself screwing that up :mad2:
 
Johann said:
Pedals, please post your results if you get to it before I do (I won't be flying until next week at least).

MAKG, the run up for this airplane is at 1800 RPM, and as you say it fails at 150 RPM drop. The drop was less than that in my run up, so I see your point. I would still want to try it though, I can see myself screwing that up :mad2:
Click to expand...
When you are out there playing try a high speed taxi down the run way, at WOT ease that mixture back see how far the RPM will drop before you feel the vibrations of a rough running engine.
 
Tom-D said:
When you are out there playing try a high speed taxi down the run way, at WOT ease that mixture back see how far the RPM will drop before you feel the vibrations of a rough running engine.
Click to expand...
If you chose to try that, please do it on a very long runway, and have someone else working the mixture and watching the RPM so you can focus on maintaining control of the aircraft.
 
Ron Levy said:
If you chose to try that, please do it on a very long runway, and have someone else working the mixture and watching the RPM so you can focus on maintaining control of the aircraft.
Click to expand...

Any private pilot should be able to accomplish that in about 5-6 seconds, with 1 eye out the window. If you can't get that done by your self get some training.
 
The best way to monitor takeoff performance is with a "G" meter, measuring "G" on the longitudinal axis and correlating that with the takeoff performance charts.

Unfortunately, that is not available in any light aircraft I know of. But it would be easy to do since (at least my Nexus 10) has three axis accelerometers.


EDIT: Hey....send money!!!!!.....2000posts
 
Tom-D said:
When you are out there playing try a high speed taxi down the run way, at WOT ease that mixture back see how far the RPM will drop before you feel the vibrations of a rough running engine.
Click to expand...

Why a high speed taxi? A full power run up will do, without the safety implications.

In general, high RPM will hide roughness, but the difference between 1800 and 2300 RPM is a whole lot less than the 800-3000 or even higher we might try on cars.
 
Ron Levy said:
Bad advice.
Click to expand...

As already noted, the OP saw the 300 drop, why would this be any different?

the OP's scan is good, already has demonstrated that they can abort the take off as they did in the first post, If you really believe that is most difficult, you should get training.
 
Jimmy cooper said:
You said it! Fast taxi is real dumb and dangerous.
Click to expand...

How fast do you have to go to demonstrate the mixture might be the problem?

You must think in real world, can you abort a take off?

All the OP must do is go full throttle, let the aircraft roll, ease the mixture back until a vibration is felt, note the RPM drop, push the mixture in and pull the throttle to Idle and exit the runway.

Could you do that?
 
Tom-D said:
As already noted, the OP saw the 300 drop, why would this be any different?
Click to expand...
A quick glance to see what it's reading while you roll is one thing, but watching the RPM to find out when it hits 300 RPM loss while manipulating the mixture with one hand off the throttle is quite another -- too much diversion of attention for too long at a critical stage of flight. That's why on the jets where they have to "trim throttles" on the role, the PNF takes care of that part after the PF advances them initially.
 
Tom-D said:
How fast do you have to go to demonstrate the mixture might be the problem?

You must think in real world, can you abort a take off?

All the OP must do is go full throttle, let the aircraft roll, ease the mixture back until a vibration is felt, note the RPM drop, push the mixture in and pull the throttle to Idle and exit the runway.

Could you do that?
Click to expand...

If you want to play it safe you can even take a second pilot to do the manipulations and observations and one control the aircraft.
 
Ron Levy said:
A quick glance to see what it's reading while you roll is one thing, but watching the RPM to find out when it hits 300 RPM loss while manipulating the mixture with one hand off the throttle is quite another -- too much diversion of attention for too long at a critical stage of flight. That's why on the jets where they have to "trim throttles" on the role, the PNF takes care of that part after the PF advances them initially.
Click to expand...

Fast taxi is stupid for any pilot but for a new private pilot extremely stupid. Simply have a good mechanic sort out the problem and they will solve the problem . Trying to solve it on the Internet is not the right answer. Any decent instructor will tell you to NEVER taxi faster than a brisk walk.....never!
 
High speed taxi has some added risk, but they are not overwhelming risks. Just remember that high speed taxi is actually low speed flight with the wheels still on the ground and that you need to fly the plane as well as drive it, you'll be ok. Every time I check myself out in a new plane, a high speed taxi test down the runway in touchdown attitude is part of the protocol.
 
Henning said:
High speed taxi has some added risk, but they are not overwhelming risks. Just remember that high speed taxi is actually low speed flight with the wheels still on the ground and that you need to fly the plane as well as drive it, you'll be ok. Every time I check myself out in a new plane, a high speed taxi test down the runway in touchdown attitude is part of the protocol.
Click to expand...

I've owned nine, mostly taildraggers , flown a mooney a lot and rented many.4500 hours logged but lots not even logged. Never fast taxied an airplane. If you rent one, be sure you have renters insurance before you fast taxi. It's a real dumb thing to do. Let a GOOD mechanic sort it out. If you have an accident doing this it could well be " overwhelming". Especially financially.
 
I don't think I'll do it.
To be honest, I never did a high speed taxi down the runway. My instructor showed me one or two for sight picture like Henning describes, but I never did it myself.
Maybe next time I'm with an instructor in a plane I'll ask to do one.

I know, I can probably pull it off, but even if the risk of messing up is small, I don't see a huge benefit. A static full power magneto check? Sure, 0 risk, but this one is a little more than 0.
Plus I think I'll have a non pilot passenger next time I go, and I already scared enough passengers for the month

Just tell me, where do I turn in my certificate?.
 
Nothing is zero risk. A high power static check has the risk of nicking the prop and setting up a crack which will lose half a prop blade in flight later. Don't do anything you're not comfortable with, but don't think because you're comfortable with it there s no risk.
 
Ron Levy said:
Sweat not -- you were faced with a critical situation, and you handled it perfectly. Press on.
Click to expand...

Thanks

Henning said:
Nothing is zero risk. A high power static check has the risk of nicking the prop and setting up a crack which will lose half a prop blade in flight later. Don't do anything you're not comfortable with, but don't think because you're comfortable with it there s no risk.
Click to expand...

True, I guess zero risk is not the right way to say it strictly speaking.
 
Johann said:
I don't think I'll do it.
To be honest, I never did a high speed taxi down the runway. My instructor showed me one or two for sight picture
Click to expand...

duplicate what you did when you found the discrepancy. You did a fine job then , why would this be any different?
 
Tom-D said:
duplicate what you did when you found the discrepancy. You did a fine job then , why would this be any different?
Click to expand...

It probably won't. Then again, maybe I end up killing the engine because I pull the mixture too much, and that would be embarrassing. I guess I could go do it at a non towered airport, but for now I probably won't, I really don't think I'll get the chance anytime soon anyway.
 
What you really should do is take that airplane out for a ride with your favorite instructor for an hour or two.
 
Johann said:
It probably won't. Then again, maybe I end up killing the engine because I pull the mixture too much, and that would be embarrassing.
Click to expand...

Why wouldn't you simply push it back in?
 
You must log in or register to reply here.
Back
Top