Diconnection of AP during GA on B737
 

Hi all!

In case you press TOGA button to activate Go Around, 1 AP trips off, so if you want AP to control the aircraft during GA, you have to have 2 APs engaged. So my question is what is the reason for this, why AP trips off during GA?

Thanks for replies
Radek

The reason you have two engaged on approach is because it is a duplex system. Two radios listening to the ILS with two a/pilots with two power supplies monitoring each other.

If the two channels differ then the system drops out with a warning as it is not safe to continue.

Once you have gone around then there is no need for two channels any more as there is no longer a radio signal to be monitored, one channel is sufficient to maintain heading once selected, and control the pitch and speed bugs.

So why does a single channel approach always have to lead to a manually flown go around? Surely the single autopilot could remain engaged to fly the manoeuvre?

Could be a go around because of a faulty autopilot prior to DH. ;)

Nobody knows?:( OK I think that there is a little misunderstanding here. Probably my mistake, so I will ask again and I think better.

I fly an ILS approach with 1 AP engaged. I press TOGA button and the AP trips off:eek: Why is this? Why it cannot stay engaged and fly the GA for me?

Thanks for help, guys.

The answer you're looking for is probably a complex one. I'll have a go at it though.

One of the situations that trips the A/P to off is : TOGA button pressed when below 2000ft on the RadAlt. This means that the situation you describe will always take place during an approach.

I'm guessing that the design philisophy behind this is as follows. With two A/Ps engaged, the system knows that there is no discrepancy between the two systems and can therefore safely allow one of the A/Ps to fly the GA. With one A/P engaged there could theoretically be something wrong with the A/P that has not yet been detected. The system itself cannot figure this out by itself as the second A/P is switched off. So if you would allow the A/P to fly the GA you are setting yourself up for a possible flightpath deviation due to an A/P error, with no backup system except for the pilot!! Also below 2000ft there is no big margin of error should a pilot be slow to recognise the problem. Because of this the A/P trips off on pressing the TOGA button, leaving the pilot in charge of configuring the aircraft for a GA with the option of A/P engagement when climbing again through a certain altitude. Say you're pressing the TOGA button at 1600 ft, you could then safely engage the A/P again as soon as the aircraft is trimmed out again and climbing.

So the answer would be: no backup and no room to manouver in case of an A/P problem. That's just my guess though, someone else may be more knowledgable than me!

I've been asking myself the question for a long time. The answers provided here aren'T really enough to convince me - just as an example both the 767 and 747 are able to fly a GA with only 1 AP.
The point that the AP won't "see" that it deviates from its flight path doesn't really convince me either - think about GPS approaches using VNAV and LNAV with only 1 AP... Or simply flying a route in LNAV.

Regards,

Mark

Hello everyone,

Just another angle to all this. The autopilot go-around is not available until flare mode is armed. So, the answer must be in there somewhere.

Regards.

Maybe is something related to Fail passive Ap's Vs Fail operational Ap's ? :confused: