Thoughts about air-ground relays for AFDS

[Hardy Heinlin]

Good morning,

as we know, for certain mode changes the AFDS mode logic needs to know whether the aircraft is on the ground. For some of these tasks it checks the radio altitude, e.g. it inhibits TO/GA engagement when below 5 ft RA for 2 seconds. But for the change from FLARE mode to "nose-wheel-down" pitch mode (FLARE word blanked) it checks some air-ground relays. In PSX, and on the real ship (if I got it right), it checks those relays which are powered by the overhead CBs H8, F22, F23, and which are in air mode when not powered. The AFDS senses "in air" when any of them is not powered. (In case of a power failure, it's better to be in air mode than in ground mode.)

Now I'm wondering if the autoland self-tests shouldn't include a check of the air-ground relay system. If I pull H8, or F22, or F23, all three FCC single channel and multichannel tests are still passed. But the FLARE mode will not terminate on the ground because the AFDS remains in air mode. Perhaps this is not so dangerous, but the nose wheel touchdown may be harder ...


Regards,

|-|ardy

[John H Watson]

Whilst it makes sense to tell the AFDS system it is on the ground, there is no mention of an input from the air/ground relays to the AFDS system for this function. The only reference to the AFDS in the air/ground wiring schematics is "to disconnect maintenance monitoring circuit in air mode"

According to my books, nose let down occurs with the radio altitude less than 5 feet and after the flare pitch command is less than 2 degrees. Something else must be controlling the lowering of the nose prior to this. Ref: Instr. Book 15 - Page 140

[Hardy Heinlin]

Something else must be controlling the lowering of the nose prior to this.

Interesting. It must be the RA too then, if it's not gear related. What else should be able to provide ground proximity data if not the RA?

Maybe at 5 ft the AFDS starts the lowering from >4°, and when below 2° (and no climb above 5 ft), it lets the nose down to the ground ...

[John H Watson]

What else should be able to provide ground proximity data if not the RA?

Not sure what you mean. If the RA is inop, then autoland isn't possible.

The air/ground system has inputs into the FMC and the SRMs (Stab Trim Rudder Ratio Modules), but I don't know if air/ground status is sent from these boxes to the FCCs. But if the AFDS doesn't use air/ground for rollout, then is it needed at all?

[Hardy Heinlin]

What else should be able to provide ground proximity data if not the RA?

Is this a grammatical phrase that cannot be translated in English? :-) I mean to ask: Isn't the RA the only thing that is able to provide ground proximity data? Or: What else, except for the RA, should be able provide ground proximity data?

But if the AFDS doesn't use air/ground for rollout, then is it needed at all?

As discussed above, it needs a point where it can safely terminate the flare mode and start "nose let down". If it gets no PSEU data, it must use RA data for this, I think. Flare mode puts the nose way higher than 2°.

[John H Watson]

I think it was the expression "ground proximity" which confused me. Proximity means nearness. Weren't you looking for a physical touchdown signal?   

As discussed above, it needs a point where it can safely terminate the flare mode and start "nose let down". If it gets no PSEU data, it must use RA data for this, I think. Flare mode puts the nose way higher than 2°.

Agreed. After 5 feet RA and after the plane has physically touched down, does the inertia of the aircraft (downwards) or decay in airspeed and thrust naturally push the nose down to 2 degrees?

[Hardy Heinlin]

No matter what the aircraft would do aerodynamically on its own, the AFDS controls the pitch dynamically. It does not apply a static hydraulic pressure or elevator angle and then accept whatever happens. So the AFDS needs to know the distance to the ground in order to decide when the nose should no longer stay above 4°.

I think it was the expression "ground proximity" which confused me. Proximity means nearness. Weren't you looking for a physical touchdown signal?

I'm looking for an input that tells the AFDS whether the aircraft is somewhere between 0 and 5+ ft.


So the grammatical phrase was correct? Another example: "Who should be able to save the earth if not Superman?" Or: "Who, if not Superman, should be able to save the earth?"

[John H Watson]

No matter what the aircraft would do aerodynamically on its own, the AFDS controls the pitch dynamically.

Good point. And I guess you couldn't leave nose let down until the elevators can no longer keep the aircraft in the air (due to decaying airspeed). i.e. Once you've stalled the elevators, you have no control in letting the nose down.
Perhaps in flare mode, the elevators do start to command an initial nose down some time after 5 feet RA?

Yes, your grammar is fine (AFAIK). 

[Hardy Heinlin]

Maybe it's the same below-5-feet-for-2-seconds condition that is used to start the TO/GA inhibit.

[Will]

So the grammatical phrase was correct? Another example: "Who should be able to save the earth if not Superman?" Or: "Who, if not Superman, should be able to save the earth?"

Both of these are fine, although I would add a comma in the first sentence: "Who should be able to save the earth, if not Superman?"