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-   -   Airbus ALT* (https://www.pprune.org/tech-log/640564-airbus-alt.html)

Newcomer2 20th May 2021 16:42

Airbus ALT*
Quick question: in my manual it says "do not select v/s or push to level off while in alt* "
Could someone explain why? All I can think of is that the aircraft may overshoot the altitude in that case (if so, why?), but is there another reason?

vilas 20th May 2021 16:46

ALT* is a capture phase with the set altitude. VS 0 or push to level is just that. It has nothing to do with altitude. If disturbed it will maintain VS0 at different altitude.

Newcomer2 20th May 2021 16:57

Ok, but my manual doesn't mention V/S 0, just V/S. From what I understand you're not supposed to change the mode while in alt*. Let's say you're in OP climb, and just before reaching your target altitude you want to reduce the rate, you select V/S+500 without noticing you're in alt*. What can happen? Is there a risk of overshooting the altitude?

Jesse Pinkman 20th May 2021 18:00


V/S 0 is just another way of saying Push To Level. Pushing the V/S toggle demands V/S=0.

Nightstop 20th May 2021 18:24

Pushing V/S Zero is an excellent method to reduce an excessive rate of climb or descent rapidly when you expect a TCAS RA will occur during ALT*, due traffic above or below. Once a reasonable rate is achieved, rearm ALT.

tubby linton 20th May 2021 18:39

Alt* is what killed Nick Warner and others in an A330 as it had no speed protection.Pushing V/S=0 just shows a lack of SA of the traffic around you. Having flown an Airbus that had an impressive climb performance (A306) it was standard to minimise the climb rate well before the cleared altitude was approached.

vilas 20th May 2021 18:43


No! During ALT* leave it alone it's too late. It's a capture phase where the rate of CLB or DES is flattening out. Even VS0 is not instant but gradual. You are likely to mess it up. When in a hurry people have played piano on the FCU. Pressing EXPED instead off ARM(ILS) then not knowing how to come out tried to press EXPED again but this time pressed ATHR causing THR LK, then trying to remove THR LK pressed ID on thrust levers driving thrust to CLB and busting speed and also causing GPWS terrain. Both seats were occupied by Training Captains.
TCAS does it's job well don't do anything last minute.

Nightstop 20th May 2021 18:52

Both seats were occupied by Training Captains.
That just goes to show that those who canít do, teach 😉.

vilas 20th May 2021 19:08

" those who have stopped learning have taken to teaching". Oscar Wild

Nightstop 20th May 2021 19:19

No offence meant. This forum is not Bluecoat.

FlightDetent 20th May 2021 19:34

Originally Posted by vilas (Post 11048240)
No! During ALT* leave it alone it's too late.

There is a chance. Actually, V/S = 0 is a way out and a decent choice. With 4000 fpm or alike it comes WAY too early and causes a whole lot of trouble.

Other methods, as you suggest, are actually futile and create even more confusion. V/S=0 is the way to go, unless you disconnect and remove FDs. In the heat of the moment, many forget the latter and you can guess the rest. For 1000-leveloff RA encounters without FDs, the statistics already spoke.

V/S = 0 is a trustworthy tool to get out of unwanted ALT*.

tubby linton 20th May 2021 20:49

Do you remember the Armavia accident at Sochi in 2006 Flight Detent? The crew used the push to level off function and then engaged open climb within 30secs The accident report stated that by doing so the autopilot is allowed to use a more aggressive maximum value of 0.3g rather than the usual 0.15g.
The report is the only place I have ever seen that written but I have been more reticent to use this function after I was made aware of it.

Sergei.a320 20th May 2021 22:09


NO, you will capture Alt* as soon you have it in blue(magenta) on your FMA. If you overshot your FCU selected Altitude with use of V/S , Alt Blue(armed) will dissapear from FMA..

Sergei.a320 20th May 2021 22:17


I absolutely agree with you! It's worth nothing to push to lvl off, rather perform RA memory item ...
But if you are not sure where your VS 0 knob is, dont touch the "piano"...

swh 21st May 2021 00:39


If you want to reduce the V/S in ALT*, pull open climb. You will see a brief OP CLB then ALT*, with ALT blue, ie protected from a level bust. The second ALT* will be at a lower V/S as the climb to level off transition (ALT*) is recalculated.

Mrmartinnico 21st May 2021 02:27

Unfortunately, it does not have to do with "over/under shooting the desired FCU alt", according to the FCOM:
"The ALT* and ALT CST* modes have internal V/S guidance that is a direct function of the difference between present altitude and the altitude target. The system switches automatically to ALT (altitude hold) when the altitude deviation becomes less than 20 ft." Taken from Autoflight, Flight Guidance, AP/FD Vertical Modes, Alt Acquire Mode.

Therefore, pushing V/S to level off, would command a level off (V/S 0) at the present altitude, instead of actually capturing the FCU ALT Target. On the other hand, selecting a V/S, eg: 100ft/min would only revert the vertical guidance to V/S + 100, which wouldn't capture the FCU altitude either.
Hope I'm not mistaken.

FlightDetent 21st May 2021 02:34

tubby linton

(edited my original post)

I do very well, and the similarities with GF072 at Bahrain too. Also what geometric path of ALT* did to the A330 you rightfully mentioned. Definitely not an advocate V/S=0 for a makeshift go around, or doing 360 at low level instead of performing a proper one. Tons of Airbus presentations on that topic. Some tools at our disposal only need to be used when applicable and not when inappropriate.

True enough, the only other use case for V/S=0 I can think of is circle-to-land at 100 above MDA, really.

When gods of physics plays against us, with 4000 fpm climb rate you get ALT* at 2800 to go. Speed starts to drop, and you get TRAFFIC TRAFFIC against intruder 1k above the cleared level.
  • Push V/S=0, observe the nose drop with 0,3g unwinding VSI towards a reasonable value, dial +1500. Simples: one press and half-a-spin. Steady and predictable state results, neatly organized.

Reports of colleagues following the "outside of red arc" above the cleared level, towards the RA's target miss of 650 ft, are well known all over the world as a result of the full takeover option with their mind lagging behind.

The explanation behind OP CLB method sounds interesting, although my mind is playing tricks OP CLB is exactly what does not work.
Still feels unnatural at the first touch, OP CLB is something I'd normally associate with an unrestricted, more aggressive climb. VS=0 to interrupt the climb. The intuitiveness is opposite.

I did learn here about a trick where V/S=0 saves from further embarrassment if you got :E OP CLB instead of GS*, sounds interesting but for now it is an internet tip - so..

Any particular I painted myself into a corner stories about V/S=0 you're willing to share? Strictly anecdotal, of course. Do not get me wrong, I'd much prefer to heed the warning!

vilas 21st May 2021 03:35

in my manual it says "do not select v/s or push to level off while in alt* "
​​​​​​when your manual specifically forbids something you don't do it because someone told you to. What happens if you did the answers are at variance. Discussion is OK but before actually doing it ask your company, may be Airbus through techrequest on Airbus world. I didn't suggest anything I just quoted an incident which was totally uncalled for but happened and was serious.

pineteam 21st May 2021 03:36

Originally Posted by Nightstop (Post 11048234)
Pushing V/S Zero is an excellent method to reduce an excessive rate of climb or descent rapidly when you expect a TCAS RA will occur during ALT*, due traffic above or below. Once a reasonable rate is achieved, rearm ALT.

I totally agree with you. This is common sense. Why would let have an RA if you can avoid it. This is not an escaping manoeuver just pilot technique to avoid triggering a RA. We had a case of TCAS RA both company traffic were on ALT*. I used vertical speed zero several times when I have excessive ROC/ROD especially on the NEO aircraft. It helps avoiding RA. It’s a mandatory occurence report here.

Also if you are doing NPA and you reach your FDP and you still on alt* You can not select FPA or VS. Pressing vertical speed zero allows this. Of course you can select an higher altitude then FPA. Both works.
I’m really curious to know why your company won’t allow to use vertical speed zero when in alt*.

FlightDetent 21st May 2021 04:50

Though it's probably better formulated as a choice of technique to unlock ALT*. Not a decision between V/S=0 and nuisance RA, black-white.

Never thought of the FDP / ALT* combination as such but in retrospect, that's what indeed happened many times. Often the last moment both pilots and the FDM recordings will agree upon :) when debriefed.

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