3 CMD switches, 747

[Tonic Please]

Hello,

Mere PMDG 747X user, but having searched for 30 mins on PPRuNe, Google and the books supplied (PDF), I just can't find an answer:

What is the difference between the three CMD switches for AP functionality?

I understand the roll modes, the FCC connections, the fact all 3 are used for precision on approaches (due to rudder input) but simply: why select L C or R after take-off, for example? Is it simply: they all do the same thing until APP mode is activated then it needs all three? So, next question would be: why three? If one provides roll and pitch during regular flight phases, and another provides yaw (which one?), then why have three and not 2? I just don't see how the numbers add up and it's somewhat frustrating me that I can't find a clear answer anywhere.

[Cough]

I recon the L C R selections depend on which IRU and FCC you connect the A/P to. eg, if the C IRU fails, you may want to select one of the others...

[LeadSled]

Tonic
Some earlier Classic B747 did, indeed, only have two autopilots.
Except for a coupled approach, consider each A/P as a separate and independent system, all have the same functionality.
When coupled and all engaged ( I forget the height this happens) the system logic has the three systems cross voting, if one is out of whack, it will disengage.
The memory aid: Three up, Cat 111, two up, Cat 11.

[main_dog]

Technically no difference provided all FCUs, ADCs and hydraulic systems 3, 1 and 2 working. My company however requires that the autopilot in use reflect the transponder selected due to ATC altitude information, so if the RT xpndr is in use (probably FO flying) AP R should be used, if LT xpndr selected (typically CPT flying) then select AP L or C.

[roundsounds]

The L and C autopilot selections reference the Capts flight instruments for speed / altitude data and the R autopilot references the FO instruments. The APP mode arms all 3 autopilots when selected and following a successful integrity check below 1500' Rad Alt, they become active and introduce rudder input. This website reproduces some of the Boeing FCOM info and might help you:
http://meriweather.com/flightdeck/

[B-HKD]

The APP mode arms all 3 autopilots when selected

Correct. This is a customer option which most B744 operators have purchased. On the other hand I know of one airline (United) which did not purchase this option. Thus, after pressing APP, the other 2 autopilots must be manually selected.

[Semu]

On some classic installations, especially in later years, the choice of which autopilot to engage was driven by the expectation of which one was most likely to work right.
One of the most - interesting- landings in my career was an autoland on a dual channel classic, due to a management mandate to start using autoland. It was even worse than my landings. Turns out, it may have been the first autoland in a decade for that aircraft.

[SMOC]

If the 747 was built new post circa 1994 it would have a single A/P push button, the triple A/P push buttons were just a carry over from the triple* switches of the classic. * if fitted.

All the A/Ps do the same job, as stated there are few nuances associated with each one.

[JammedStab]

Originally Posted by Tonic Please

So, next question would be: why three? If one provides roll and pitch during regular flight phases, and another provides yaw (which one?), then why have three and not 2? I just don't see how the numbers add up and it's somewhat frustrating me that I can't find a clear answer anywhere.

Three Autopilots equals Fail-Operational CAT IIIB ILS approaches with no decision height and 50 meters visibility(assuming all other required approvals from CAA etc and ground equipment allow this type of maneuver). If one is inop, then two Autopilots equals Fail-Passive with a 50 foot DH and higher vis requirements. Both situation allow autoland and rollout. Only one Autopilot(could be any one of the three) allows a coupled approach with no autoland allowed and Cat 1 minimums. A matter of redundancy.

So what happens if one of the autopilots fails on the CAT III approach? It depends where it happens. If below what is called an Alert Height which is 200 feet radio altitude, we continue and land. In fact, I don't believe that there will be any indication of the failure while airborne below the Alert height.

Whichever of one of the three autopilots is individually selected in normal flight will only provide pitch and roll inputs. The three of them together provide rudder input as well once all three are engaged which will happen automatically without pilot input on an ILS approach around 1500 feet. Of course that is necessary for crosswind correction for autoland and helps in an engine inoperative situation for approach and go-around(although one has to be ready to apply rudder when the aircraft automatically goes back to single autopilot operation). In other words, the pilot always selects one autopilot on. The rest engage automatically during the approach when appropriate conditions are met.

[Denti]

Interestingly enough, the baby Boeing needs only two autopilots for the same functionality. Although the second autopilot has to be engaged manually, which is possible after the approach mode is armed.

[nicolai]

Originally Posted by Semu

One of the most - interesting- landings in my career was an autoland on a dual channel classic, due to a management mandate to start using autoland. It was even worse than my landings. Turns out, it may have been the first autoland in a decade for that aircraft.

Was there no requirement to perform an autoland if operations had not required one in some reasonable interval?