Altitude pre select not responding on occasion

So sometimes when I am descending to a certain altitude and I set it to my A/P (example 3000ft) my aircraft keeps on desending after it reaches 3000ft. When I realize this I double tap on the ALT A/P and there is a rapid climb back to 3000ft. Anyone know why this is happening?

Are you looking at your MSL or AGL altitude?

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MSL altitude. This happened at PHNL so its pretty much sea level.

Take a look:

That is weird. How far does it drop beyond the point you set it to go at? Does it keep descending until you touch the ground?

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Yes it keeps desending until I turn on and off the A/P againl

Huh, pretty weird. Check the post above stated by Starley, might have some key information. What aircraft where you flying?

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Well they said it was to 4000ft but that wasn’t the case for me. I was using a 747-400

But the altitude set clearly isn’t the reason. I still suggest you take a read through.

I did but I can’t find key info.

Can you reproduce and record it? That would be very helpful.

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Im sorry. I didn’t record it. But next time it happens I will be sure to record it.

Here is some more info. 180 KNOTS A/P set to 3000FT turning into the localizer and my altitude went down to like 500 ft.

Turning at 180 KIAS in a (possibly heavy) 747.

If your speed [in conjinction with flaps] isn’t enough to maintain altitude during turns (depending on the bank angle) you can lose altitude.

I think I was with flaps 20 at 180 IAS. But it wasn’t very heavy because I was about to finish my flight.

Also let me add that I was manually turning so I was keeping a steady bank angle.

I don’t know what else to offer. Over the course of thousands of flights (I’m sure you’re the same or more than me), I’ve never had a loss in altitude that was not caused by speed, bank angle, etc. I’ve never had the AP just fail to respond. If you aren’t able to repro I would just not worry about it.

Try the same thing again, same set up, etc, and record it. If you’re able to repro, then there would be something to work off of. If it doesn’t happen again, it was probably just some variable for that one flight which isn’t going to be a recurring issue.


Ok I will be sure to record it if it happens again and will notify you guys.

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Your favorite route perhaps?

You think I should start a new thread? ;)