ALT CAP question

[Diesel]

Any bright sparks out there tell me what, if any, speed protection exists while ALT CAP is anunciated on the FMA on a 767?

Does the autopilot pitch for the level off with no regard for speed or is there some speed input to the level off manoeuvre?

Thanks in advance

[757jetjockey]

I believe it will have something to do with which mode the A/T annunciates:

SPD: it will control speed to whatever it was when ALT CAP was annunciated, this is always an IAS number too, which is annoying sometimes when your flying Mach No and getting step climbs

EPR/N1: it will maintain the reference EPR/N1 setting, this is a risk with low level ALT CAP, as you can level off with Takeoff EPR/N1 still set and very rapidly accelerate leading to bust flap limits etc if your not aware of it..

As for low speed protection, ie if you were climbing near to stall speed or something like that and pulled back just before it went into SPD / ALT CAP, never been in that situation, so don't really know, interesting question though

757JJ

[Diesel]

Ok

So if you take off and get alt cap, does the autothrottle speed mode affect the level off?

Let's say the speed is decreasing and the autopilot is in ALT CAP, can you do anything other than disengage the autopilot to prevent the speed decaying any further? Would selection of SPD on the MCP make any difference given thrust is at maximum CLB anyway?

[deltahotel]

I'm hoping the 767 FMC is the same as the 757!! If the Alt Cap happens shortly after T/O (before Clb is selected) the mode will be EPR/Alt Cap, as 757JJ says. in this mode the A/T mode is EPR controlled at the derated T/O figure (1.4 - 1.5 ish) and the ac will accelerate level like a scalded cat.

1. Select CLB thrust
2. Set VRef 80 in MCP window
3. Press SPD button on MCP

This will allow the ac to go to VRef 80 and no further while you clean up on schedule.

If you've already selected Clb it will open as SPD/Alt Cap (live speed mode) and you can wind the spd up to Ref 80.

If it's in G/A, that will produce Spd/Alt Cap - again a live spd mode.

Hope this helps

DH

[763 jock]

Autothrottle changes from EPR/N1 to SPD at the same time as ALT CAP occurs. Speed window opens at the actual IAS that was being flown when ALT CAP occurred. This is assuming you were climbing in VNAV rather than FLCH.

The autopilot pitches to achieve a smooth level off depending on rate of climb, the autothrottle provides thrust to fly the commanded speed. If you're slow, just increase the commanded speed.

The only time I've ever seen the speed low was in wave activity.

[eckhard]

Wasn't there a Britannia 757 that got a bit slow during an early level-off on a SID a few years back? I think they had such a high rate of climb that the ALT CAP mode engaged several hundred feet before the pre-selected altitude and somehow the thrust reduced to the level-flight value required for the current speed even though they were still climbing for several seconds. As a result the speed reduced to an uncomfortably low value. I seem to remember the CAA or AAIB investigated.

My memory is not what it was so all the above may be wide of the mark. Anybody else remember this incident?

[Diesel]

Deltahotel

I would generally agree however..."in this mode the A/T mode is EPR controlled at the derated T/O figure (1.4 - 1.5 ish) and the ac will accelerate level like a scalded cat."....is this always true?

If I take off with full power (say due MEL reqrmnts) get VERY early alt cap, then select CLB as you describe, the very large redution in power seems to leave the aircraft pitching for an ALT CAP profile it can no longer achieve.

Suddenly, rather than the excess energy situation you describe, there's not enough! You already have full CLB EPR selected, so the autothrottle can't help you, and the autopilot is pitching for a level of that's too far away now you no longer have T/O power....

Anyone else seen this? Is there anything that can be done other than disengage the autopilot and start again?

Diesel

[Tight Slot]

Pushing Flight level change will. That will re-compute an ALT CAP for the given energy.

[Diesel]

Tight slot

Thanks for that. Yes I have found that this works. Question is if you leave the automatics in ALT CAP, will the aircraft continue to pitch for the level off which is too far away, or protect the speed?

Diesel

[Tight Slot]

On the 75 (or76..) the aircraft will only think of the pitch to gain the pre set ALT CAP altitude. Ie - if you close the thrust levers ( sorry - im airbus now!) on an alt cap, the aircraft will still command the given pitch to get the altitude. Thus ending up in tears.

No speed protection as far as I'm aware.

Regards

[Diesel]

that was my suspicion, thanks

[heavyg]

It actually depends on the 'customer option' ordered from Boeing. On our fleet the majority of the 767s do not respect airspeed during 'ALT CAP'...however; a few of our newer 767s do provide speed protection during the 'ALT CAP' event. So I guess both answers could be correct depending on the options ordered by your operator.

[Diesel]

thanks for that. Will find out what we have.

diesel

[757jetjockey]

On the 757/767 we operate, We have auto climb thrust enabled on most of the fleet.

In the case of a low level ALT CAP we can use any of the following, depending on what we want to achieve.

1. VNAV
2. FL CH
3. CLB Pwr, set desired speed in MCP window, then SPD.

As far as low speed protection applying I have never been that far back down the speedtape to have worried about it. However, proper briefing when this is likely to occur will bring it to both pilots attention.

One method I occasionally use in the event of a Low Level ALT CAP occuring is to select VNAV at a lower than 'normal' height. Our standard accel alt is 1500'aal, but going out of Orlando for example, where an iniial clearance is to 3000ft, I will brief the selection of VNAV at 1000'aal to avoid said problem, most trainers I have flown with are fine with it and it has never been criticised before.