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Question On The MD80 Series Autopilot  
User currently offlineOli From France, joined Jun 2005, 25 posts, RR: 0
Posted (10 years 6 months 1 day 17 hours ago) and read 6707 times:

Hi all,

On the MD80, you have to arm the altitude after you select it on the MCP. I would like to know why that is, since it seems pretty dangerous to me, the plane can fly higher or lower than what you requested if you forget to arm the altitude (even if there is an alarm).
Is is because it was technically impossible when it was designed (I do not really believe in this one).
Is it a pilot request ? If so, why ?

I also would like to know if pilots find this irritating or it does not matter to them ?



5 replies: All unread, jump to last
User currently offlineMandala499 From Indonesia, joined Aug 2001, 7528 posts, RR: 76
Reply 1, posted (10 years 6 months 1 day 15 hours ago) and read 6694 times:

Typical Boeing and Airbus answer....

That's why it's a McDonnell Douglas plane...

Just kidding...


When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2620 posts, RR: 25
Reply 2, posted (10 years 6 months 1 day 12 hours ago) and read 6656 times:

I presume this is done as a safeguard against inadvertent altitude selection in IFR controlled airspace. It's quite normal to have to separately engage altitude select mode, whether in an MD, a Boeing or an Airbus. If you are already in altitude select mode then simply adjusting the altitude will change the reference with no need for a second selection.

The AP also needs to know how you are going to achieve the altitude change. Going from altitude hold to altitude select enables other modes, such as IAS or VS hold. On modern autopilots you can go straight to ALT SEL and IAS SEL using the Level Change mode (FLCH), which also puts the A/T in the correct mode too. Finally, you also have to provide a clear cue for the FMC to work with the new altitude.

The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineOli From France, joined Jun 2005, 25 posts, RR: 0
Reply 3, posted (10 years 5 months 4 days 16 hours ago) and read 6447 times:

The reason the MD system surprises me is that, on a Boeing, you select the altitude you want to fly at, then you select how you want to clime/descend (V/S, FL CH..).
In the MD, you have to select the altitude, arm the altidue, and then select how you want to descend/Climb. If you forget to arm the altitude, even though you selected it, you plane will go on climging, descending (there is a warning though).
I was wondering the reason for this extra step.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2620 posts, RR: 25
Reply 4, posted (10 years 5 months 2 days 20 hours ago) and read 6366 times:

If you go back further to the 747 Classic AP/FD you'll find three step logic there (select altitude, select vertical mode, select ALT SEL). Sometimes if you change vertical mode ALT SEL will trip off and have to be reselected. Easy to miss that too, unless you check the FMA.

The digital autopilot on the MD80 series does all kinds of things automatically, such as switching to ALT HOLD if you select zero V/S for example, so it's fairly user friendly. I've only ever "flown" MD82 and MD88 simulators, but I don't recall having to separately arm the altitude. Maybe I just didn't notice doing it. An MD80 pilot might know the answer.

The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 5, posted (10 years 5 months 17 hours ago) and read 6283 times:

Having to arm the selected altitude is a customer option I think.The MD-81/82/87 I fly for SAS arms the selected altitude automatically.This happens approx. 1 second after the altitude has been selected.Then I can choose mode of vertical movement: either V-nav,vertical speed or IAS in conjunction with appropriate thrust increase/decrease.

"Luck and superstition wins all the time"!
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