Altitude pre select not responding on occasion

Nicolas_Betancourt · February 21, 2018, 9:53pm

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?

Starley · February 21, 2018, 9:57pm

Are you looking at your MSL or AGL altitude?

Nicolas_Betancourt · February 21, 2018, 9:58pm

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

Starley · February 21, 2018, 10:00pm

Take a look:

Royce · February 21, 2018, 10:00pm

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?

Nicolas_Betancourt · February 21, 2018, 10:01pm

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

Royce · February 21, 2018, 10:02pm

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

Nicolas_Betancourt · February 21, 2018, 10:03pm

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

Kevin_Potthast · February 21, 2018, 10:04pm

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

Nicolas_Betancourt · February 21, 2018, 10:05pm

I did but I can’t find key info.

Starley · February 21, 2018, 10:07pm

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

Nicolas_Betancourt · February 21, 2018, 10:08pm

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

Nicolas_Betancourt · February 21, 2018, 10:10pm

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.

Tim_B · February 21, 2018, 10:15pm

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.

Nicolas_Betancourt · February 21, 2018, 10:16pm

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

Nicolas_Betancourt · February 21, 2018, 10:17pm

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

Tim_B · February 21, 2018, 10:21pm

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.

Nicolas_Betancourt · February 21, 2018, 10:22pm

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

Scott_Kazoo · February 21, 2018, 11:58pm

Your favorite route perhaps?

Tim_B · February 22, 2018, 12:37am

You think I should start a new thread? ;)