Transition from autoland to no-autoland

[Hardy Heinlin]

Good evening,

I just noticed an interesting effect which is a consequence of the bus isolation logic.

This is a good test situ: 03 Approach 016 - Rome - TCAS fault.situ

Wait 5 seconds.

Activate Severe Compressor surge/stall 3 & 4 (severe means: surge and flame-out).

Result:

AC busses 3 & 4 fail for a fraction of a second until the autoland bus isolation closes so that they are repowered via the sync bus from AC busses 1 & 2. During this power transfer some sections of the flight deck lighting dim to 12 v emergency power, some sections get dark completely (watch the whole deck). And because DC bus 3 is momentarily unpowered as well, the autobrakes disarm. They can be rearmed immediately after this short break.

I don't think the bus isolation system can foresee the power loss and close the BTBs before the actual power loss in order to provide a "no break power transfer". So there must be a momentary power loss on AC busses 3 & 4 (and the related DC busses) as it happens in the simulation.

But I'm wondering whether the autobrake system on the real deck will disarm promptly within a fraction of a second, or if it's able to keep its armed status for a moment when DC power is removed.


Cheers,

|-|ardy

[United744]

How long is the actual period of power loss (not from the moment of power failure, but from when systems start reacting to the loss)?

The solenoid probably doesn't react that quickly to the actual loss of power. If the power is lost for 1/2 sec then I'd say it would disengage, but if the power failed for 1/10th sec then I'd guess it depends upon the switch in question (it may or may not disengage).

In addition, the arming logic itself might cause a disconnect if it sees power failure?

[John H Watson]

Lots of factors here.
Strong spring on the selector.
No capacitor (AFAIK) on the selector to hold a charge.
TRUs probably have big capacitors to hold charge (but all the DC systems are still trying to draw power), etc.

Is PSX programmed for "worst case scenario"? i.e. Do we give the pilots a semi-optimistic outlook or prepare them for the worst? (So they are ready for anything that might happen)?

[Hardy Heinlin]

Is PSX programmed for "worst case scenario"? i.e. Do we give the pilots a semi-optimistic outlook or prepare them for the worst? (So they are ready for anything that might happen)?

As the solenoid releases instantly in PSX, PSX simulates the worst case, I would say.

[Jeroen Hoppenbrouwers]

DC electronics must typically survive 200ms power interrupts unharmed.

[United744]

Hi,

How is the resiliance to source switching/power interruption actually achieved? Is it simply a case of using large capacitors in the power supplies or...?

[Jeroen Hoppenbrouwers]

Yup.

And then you get the issue of inflow rush when you switch the thing on.  Normal bi-metal circuit breakers won't even blink, but the modern electronic breakers such as in the 787 immediately trip if you suck 15 amps for 5ms on a 2 amp circuit.


Hoppie

[John H Watson]

The 744 computers will have power supplies with normal size capacitors, but the 744 tries not to rely on these when switching between power sources. If power switching is done normally, the aircraft will actually put both power supplies on the same bus at the same time for a brief period ("No break power transfer"). To put two sources on a bus at the same time, they must be in sync (frequency and phase).

However, in some circumstances, this cannot be done ("break power transfer"). This will be the case in Hardy's scenario. Dead busses will have to be sensed (as such), then switching will be commanded. Lights will flicker. Perhaps even flags will appear on instruments.

From past experience, results can be variable. e.g. an Audio Control Panel will go dead and you will have to reselect buttons. In other cases, the selections remain. My personal preference would be for the former. If you expect the worst, you are prepared for it (and know what to do about it). In the case of an Audio Control Panel, you don't get any messages on EICAS and it can be unnerving if you try to contact someone on the nosewheel and can't even hear your own voice (sidetone).

Rgds
JHW

[Jeroen Hoppenbrouwers]

Actual electronics often also have a brown zone. Below, say, 80ms everything is fine. Above 150ms everything is also fine, as in, everything is truly dead and thus will come back up. But in between 80ms and 150ms, half the box dies and the other half sort of survives and both halves do not find each other. You now have a box that is inop and needs a proper CB cycle to come back.

You then need to go for broke and include a suicide circuit: it artificially shorts the power (or grounds all reset pins) at the 80ms limit even if there still is sufficient power for some/most of it. Better be truly dead and resurrect, than to waver around in the zombie twilight zone.

Adding larger caps is not always a viable option.


Hoppie

[Hardy Heinlin]

The ACPs in PSX can hold their DC power for 333 ms. If the power break takes longer than 333 ms, the respective MIC light will extinguish.

For this reason, the observer's ACP, for example, will not fail when in the above scenario  engine 1 fails instead of engine 3.

I think the 333 value (a third of a second) is based on observations on the real deck.


|-|ardy

[United744]

Great explanations! Thank you!