grockpilot
Posts: 16
Joined: Sun Jul 22, 2007 3:52 am

A/P Engage

Wed Dec 19, 2007 8:58 pm

Hi,
Now I know some of you are gonna think this is a national security threat or something but I hope some of you will be willing to answer an innocent question from an aspiring pilot who's just curious. (Kinda sad I have to explain myself like this...times we live in huh)

I noticed on the 737s, the A/P engage has CMD A/B and CWS A/B. What is the difference? Also in the 757/767/747/777, I noticed that A/P engage has CMD L, C, R (It's also in the movie Turbulence, which sucked hard and isn't a good reference). So what exactly is the difference and what is the benefit of each?

Just curious.

Thanks,
Ray
 
avioniker
Posts: 1099
Joined: Tue Jan 01, 2002 5:38 am

RE: A/P Engage

Wed Dec 19, 2007 11:25 pm

The letters: A, B, L, C, R designate the autopilot in use. On the 37 there's two: A & B and on others there may be three: Left, Center, and Right.

Normally the pilot flying will use the autopilot on that side of the plane, A for left or B for right, during cruise.

When using the autoland modes both or multiple autopilots are engaged simultaneously to ensure safe, continued operation in the event of a failure.

Very simply put there are two modes of autoland: Fail Passive and Fail Active.

In the Fail Passive mode you have two autopilots and if one disengages, due to a fault, above a threshold altitude AGL (may be 50 or 200 ft) the pilot flying is to disengage the remaining autopilot and perform a manual landing.

In Fail Active with two autopilots and a HUD or three autopilots (depending on the certification level of the operator) the remaining autopilot(s) will be allowed to take the aircraft to a landing with close monitoring by the pilot flying. I know of no aircraft certified for single autopilot landing. (that, of course, doesn't mean there aren't any)


Don't confuse the autoland modes with the categories of IFR landing, I, II, and III a, b, and c. The requirements are different for the autopilots and the operators and the national certifying agencies also have their own certification levels and requirements.

Confused?

Good. . . There's a whole lot more information about specific autopilots and navigation interfaces and just about every aircraft out there has its own specific idiosyncrasies. Even though different aircraft may have the same box installed the software will be different for each application.

After working on and helping develop numerous autopilots over the last 38 years I feel that my knowledge only scratches the surface. I'm amazed to see what's happened in the avionics world. From my first autopilot on the C-47 to the SP-1 on the 141 to the Honeywell and Collins on the 737NG, some things remain the same but many many more have changed. Whether for the better or not is for others to judge.

   

[Edited 2007-12-19 15:33:33]
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
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Jetlagged
Posts: 2562
Joined: Sun Jan 23, 2005 3:00 pm

RE: A/P Engage

Thu Dec 20, 2007 1:54 am

An important issue to consider is that AP A and B have different data sources. AP A usually uses the same sources as the Captain's instruments/displays, AP B those of the F/O. So usually the Capt will use AP A and the F/O uses AP B to be consistent with the information presented and the selections that are made on their side.

On the 747 Classic this also applied, A for the Capt, B for the F/O. AP C (if installed) was mainly used for triple autoland, though it was a fully operable channel. On the 747-400 you will see L, R and C rather than A, B and C. AP C (C for Centre in this case) is often the one usually selected by either pilot.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
PhilSquares
Posts: 3371
Joined: Sun Mar 28, 2004 6:06 pm

RE: A/P Engage

Thu Dec 20, 2007 2:54 am



Quoting Jetlagged (Reply 2):
AP C (C for Centre in this case) is often the one usually selected by either pilot.

Funny how different each airline has their SOPs constructed. At SQ, the Capt selects L and the FO selects R with the C autopilot being left as the secondary selection if needed.
Fly fast, live slow
 
avioniker
Posts: 1099
Joined: Tue Jan 01, 2002 5:38 am

RE: A/P Engage

Thu Dec 20, 2007 4:05 am



Quoting Jetlagged (Reply 2):
On the 747 Classic this also applied, A for the Capt, B for the F/O. AP C (if installed) was mainly used for triple autoland, though it was a fully operable channel.

And there's other variations. At Pan Am it totally depended on when the plane was put in service and who spec'd it. They had planes with three AP's installed but only two engage levers; The third was a Flight Director or Model channel depending on the nav system installed. Some had three with three levers, and some had two. There were even a couple with provisions for three but the third not installed and the panel had two levers. There isn't enough memory in this server to discuss all the variations that were installed since 1969 in the '47 fleet.

 Smile
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: A/P Engage

Thu Dec 20, 2007 1:26 pm



Quoting Avioniker (Reply 1):
I know of no aircraft certified for single autopilot landing. (that, of course, doesn't mean there aren't any)

The MD-11 can single land down to CATII mins after that it must be dual land. The system is fail active.
 
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Jetlagged
Posts: 2562
Joined: Sun Jan 23, 2005 3:00 pm

RE: A/P Engage

Thu Dec 20, 2007 1:31 pm



Quoting Avioniker (Reply 4):
And there's other variations. At Pan Am it totally depended on when the plane was put in service and who spec'd it. They had planes with three AP's installed but only two engage levers; The third was a Flight Director or Model channel depending on the nav system installed. Some had three with three levers, and some had two. There were even a couple with provisions for three but the third not installed and the panel had two levers. There isn't enough memory in this server to discuss all the variations that were installed since 1969 in the '47 fleet.

Tell me about it. I have twice been involved in sourcing a MCP for 747 classic flight simulators and the number of combinations available is staggering. However all 747 classics I've ever seen have had a three channel Flight Director. Since the SPZ1 is a combined AP/FD unit (unlike older Boeings where the FD is a completely separate computer) it's only the AP functionality for Channel C which is missing if a dual AP system is installed.

I have no idea whether a dual autopilot SPZ1 is physically any different from a triple one (there are certainly differences depending on whether it is triple "fail passive" or "fail operational"). Certainly, a lot of the AP engagement logic is contained within the MCP hardware on the glareshield. My assumption has always been that the SPZ1 is basically a triple AP/FD, but sometimes only dual AP functionality is enabled.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
chrisMUC
Posts: 54
Joined: Sat Jul 22, 2006 7:47 pm

RE: A/P Engage

Thu Dec 20, 2007 3:25 pm

I can explain you the difference between CMD and CWS on classic 737s.
In CMD Mode, the autopilot flies fully managed, using the selected autoflight modes, like HDG, V/S, VNAV, LNAV, LOC, APP.
In CWS Mode, the autopilot will maintain a certain pitch or bank which are controlled by the pilot through applying pressure on the control wheel (= control wheel steering). If there are less than 5° of bank when engaging the A/P it will level the wings.
You can also use CMD and CWS the same time for the different auto pilot channels (pitch and roll). An example for this which I always use on climb out is engaging the paddle in CMD, selecting LNAV for the roll channel (or it has already been used for the FD) and not selecting a pitch mode. The reason for that is, that passing 10.000 ft we turn of the signs and cabin crew will start service, but with trolleys in the cabin we have a pitch limit of 7,5°. Using modes like VNAV or V/S there would be a risk that the auto pilot exceeds that limit.

Hope this helps, merry Christmas,
Chris
 
grockpilot
Posts: 16
Joined: Sun Jul 22, 2007 3:52 am

RE: A/P Engage

Mon Feb 04, 2008 8:33 pm

wow. thanks. yeah im more confused that before. haha.
 
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ThrottleHold
Posts: 545
Joined: Tue Jul 18, 2006 11:00 pm

RE: A/P Engage

Tue Feb 05, 2008 7:23 am



Quoting Avioniker (Reply 1):
In Fail Active with two autopilots and a HUD or three autopilots

Airbus aircraft are Fail Active with 2 autopilots, AP1 and AP2.

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