Which Autopilot CMD Switch to Use

[JulietAlphaSierra]

Hello experts of the 744, does anyone with far more experience that I have (not saying much) have any clue on which CMD button was most commonly used? I've seen a lot of operators use the L CMD for Captain flying and R CMD for F/O flying, but in the case of the 757/767, which have an identical autopilot panel design, the C CMD is almost always used, and I have seen a few 744 operators use this method too. Any clue on which one is best/realistic to use?
Thanks!
-Joey

[B767300]

It's an option on the 744 to inhibit F/D ops with the respective AP engaged. So L CMD, would inhibit L F/D, hence CMD C. But again just an airline-option (even in Aerowinx), it would make more sense to still engage the opposite CMD for redundancy so I'm not sure why it's CMD L for Capt flying and CMD R for FO flying.... Maybe just out of convenience

[Jeroen Hoppenbrouwers]

Unless the idea is that the absence of the their FD indicators is an extra assurance for the pilot flying that the thing is in CMD mode. "Don't touch me."

[andrej]

I know that UAL had this strange option. What other airline would opt for it?

In regards to what CMD to use. Unless there is SOP for that (is there?), I don't think there is any preference. Flying for Stevens VA only (PSX), departure from home base EDDK, any westbound flights CMD L, eastbound flights CMD R, and inbound legs CMD C. I think it is fair.

Serious question, would there be a negative impact on a specific CMD if that is utilized more frequently than others? Say I would only use CMD C for 20 years and not the other on a specific airframe.

Best,

[Mariano]

On our company 767s, we use the center autopilot. Two reasons (in my opinion) being 1- You can cross check that all three FCC commands agree, and 2- The center FCC can command both SAMs (Stabilizer Trim Aileron Lockout Modules) (one at a time), which is not the case with the left and right FCCs. If the center FCC is in command while linked to the left SAM and the left SAM/STCM fail(s), it can automatically switch to the right SAM (I can't recall the 744 system architecture in this area).

That being said, on our company 747s (-400 and -8s), they use the left or right FCCs depending on who the pilot flying that sector is.

Best regards,

Mariano

[Jeroen Hoppenbrouwers]

Given that computers don't stop working if you don't listen to them, I think for their endurance (MTBF) it would not matter if you use only one computer ever. However it is assurance that they are all looking OK if you switch them out regularly, like making an autoland every so often just to check for it. It won't make it less likely that one of them fails in the next second, but you won't fly around with a latent broken box very long without knowing.

The mechanical (hyd) actuators are not linked to just one computer as far as I remember so for these wearing components it should not matter which computer is in the driver's seat. They are all active all the time.

Hoppie

PS. I think that pretty much every flight has a segment with the APFD in control on three computers for at least a minute -- when in APP mode LAND3 on final (ILS) but before the disconnect. This would also assure that the three boxes are at some ok level of consciousness.

[andrej]

Thanks Hoppie.
Your conclusion is what I thought as well (i.e. no impact). Seems like Boeing updated its design and with B777 and B787 (they have 3 autopilots as well) and offered single A/P (CMD) button on each side of the MCP (for a convenience for each pilot).

[United744]

I use the option "same side CMD blanks FD", so I use the center AP.

There is another good reason to use the center AP: there is no visual representation on the flight deck of the center IRS attitude. If the center AP using the center IRS is in command, and the center IRS attitude starts to wander, you will immediately see it on both Captain and FO PFD. As both left and right IRS *ARE* depicted on the flight deck, it is a quick check to see that the two agree. If neither the Captain or FO PFD agree, then one of the *other* channels (L/R) failed, and not the center channel.

IMHO this logic makes most sense, as you can then monitor all three attitude references throughout the flight.