Airbus ALT*
 

Hello!
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?
thanks!

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.

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?

Newcomer2

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

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.

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.

Nightstop

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.

That just goes to show that those who can’t do, teach 😉.

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

No offence meant. This forum is not Bluecoat.

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*.

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.
https://www.bea.aero/docspa/2006/ek-...ek-9060502.pdf

TorqueStripe

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..

FlightDetent

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"...

Newcomer2

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.

FCOM
 

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.

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!

​​​​​​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.

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*.

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.

So we’re in ALT* and want to change the vertical speed. There’s a very simple 2 step process I’ve encountered that works very well.

1. red button…..push
2. airplane……..fly

In ALT*, Autothrust is in SPEED mode. If it does something weird, push the other red button and go on to step 2.

Arf Check Airman, don’t you know that most pilots will freak out if you do so? I don’t want my colleagues to have an heart attack on the sound of the AP tripping off. :}

Better a heart attack than a 21 gun salute.

Joke aside, although as you know, I’m all in for hand flying, in that context, pressing vertical speed is easy peasy and so far I never had an RA using this technique. With those guys who never hand fly in line, if you ask them to disconnect there is a high risk they will over react and pull some serious G force and hurt cabin crews and/or passengers. Happened before...

it has happened many times because no matter how much you love hand flying you don't have any experience of it at cruise altitudes which are in RVSM. So if it's done in panic there can be trouble. The aircraft is very sensitive due to less damping. The correction in pitch and roll has to be much smaller scale and one has to remember all that in a hurry. It's much simpler to be conventional than be exceptional.

Agree with you Vilas, at high altitude, I would only consider disconnecting if I really have to for that reason.
Even at low altitude I follow Airbus recommendation going from managed mode to selected mode first. I can’t even remember the last time I had to disconnect AP for path recovery. The few times I do is for example on A320 to get the full speed brake if I’m high on profile. :p

Airbus has an app called TCAP(TCAS alert prevention) to avoid nuisance TCAS RAs. When installed it automatically adjusts the altitude capture law to reduce ROD/ROC to avoid RA. Those interested can read an article about it in Airbus Safety First issue 13 January 2012.

I find this kind of discussion fascinating, because here we have a bunch of highly experienced type-rated pilots, all with a slightly different interpretation of how/why a feature works and what to do if it isn’t performing as you want. This is not a dig at any of you, but an example of how the UI and documentation of the same on many aeroplanes is far from ideal. Even between different operators there seems to be alternative ways of dealing with the situation.

Over in Boeing World, there is a very similar thing with ALT (no *) but it serves for capture and hold, with quite a few quirks for the unwary. There are phases of flight where pressing the brown button (does the same as the AB red one) is preferable to improv jazz on the MCP which may or may not have the desired result...

Going to respectfully disagree here.

Consider the situation where you climb into an increasing headwind at high level and experience a transient increase in airspeed, leading to a high rate of climb. If ALT* engages at this point you could be several thousand feet below your target altitude with the speed about to wash off. VS 0 is a perfectly reasonable mode to consider at this point to manage the energy situation. You can re-engage climb or select a sensible VS once the energy of the aircraft is doing what you want. Pulling for VS will just put you back into ALT*.

If you realise that this is going to happen a better way to manage the situation is to select the airspeed or Mach at something close to VMO or MMO, so that if the headwind washes off you can just go back to your original speed instead of experimenting with the low speed protections.

Good point Fursty Ferret! I have seen Vertical speed going over 6000 feet quite fast during climb on A320 Neo. It’s actually normal to see vertical speed going around 4 or 5000’/min at 250kt on the Neos especially if you don’t have the option of the Alternate Climb. Those engines are amazing. :cool:

FurstyMy comment was for routine ALT* situation. If faced with unusual situation appropriate intervention may be in order. I have no problem with that. The second aspect is about something specifically not recommended in FCOM(poster's) being routinely brushed aside by line pilot. That I definite have problem with it. There what I suggested is a safe way to go about.

:DNever heard of that option before, very grateful for you sharing the wisdom. :D

As we are discussing high climb and descents rates this is pertinent-
ICAO Annex 6 (4.4.10):
Aeroplane operating procedures for rates of climb and descent
Recommendation.— Unless otherwise specified in an air traffic control instruction, to avoid unnecessary airborne collision avoidance system (ACAS II) resolution advisories in aircraft at or approaching adjacent altitudes or flight levels, operators should specify procedures by which an aeroplane climbing or descending to an assigned altitude or flight level, especially with an autopilot engaged, may do so at a rate less than 8 m/sec or 1 500 ft/min (depending on the instrumentation available) throughout the last 300 m (1 000 ft) of climb or descent to the assigned level when the pilot is made aware of another aircraft at or approaching an adjacent altitude or flight level.

I forgot to mention that in some countries it's SOP to reduce vertical speed to 1000ft/mt within last 2000ft to level off. So the TA are avoided.
tubby
The VS recommendations differ within different authorities.

https://cimg3.ibsrv.net/gimg/pprune....639cd05bba.png

You can add Spain to the list, for manoeuvring in TMAs (simplified). At least that actually makes sense.

Once it's understood there's ONE technical specification and MULTIPLE regulatory restrictions, it becomes more apparent what a freak show this is.

Some entries in the table above are not even factual. Why is the FAA displayed as 500-1500? The threshold for traffic A flying level and B coming to 1000 above/below is 1700 fpm (35 sec above FL200) at the 1000 separation point - assuming the other traffic is there in the cross-hairs and without any gradual ALT capture of the one manoeuvring.

Technical specifications, ACAS ver. 7.0

https://cimg3.ibsrv.net/gimg/pprune....a8f774d71b.png
https://cimg4.ibsrv.net/gimg/pprune....c7b310cb76.png

Thanks for sharing FlightDetent. Can you explain the right side of the table ? DMOD?
And maybe give another 2 examples with different altitudes please.
if at high altitude the TAU is higher therefore it should be easier to trigger a TA or RA if excessive ROC/ROD compare to low altitude?

Ok, those tables are still somewhat simplified and I cannot vouch the values are truly identical with 7.1 or whatever is the latest. Secondly, I prefer not to hijack this ALT* V/S = 0 stuff, but as I already broke the first glass...

1) The right column shows the absolute (shortest) distance limit. Normally ACAS calculates the "closest point of approach" from the vector geometry and then warns at a pre-determined time threshold. That does not work at all for closely parallel trajectories, where CPA is minutes ahead but you'd be getting physically too close. DMOD (Distance-MODification) works in the lateral plane, imagine same-level traffic converging far ahead. Similar distance threshold exists in vertical plane (IIRC @ FL200+: TA=850 ft, RA=650 ft).

3) Agreed. Higher up the logic is more sensitive, asking for larger protective bubbles around respective traffic.

2) It's easy to calculate. FL160, one traffic at level, the other descending.

---
With 2000 to go before the cleared level, aiming to systematically avoid TA which is enough of a distraction.Assuming pilots reduced the rate and continued with 2000 fpm instead of 4k,---

Play around with the numbers, the final discovery is far from surprising. The original guidance provided from Airbus to its pilots (different from, or badly misquoted in table at post #35) is optimal.

• when actually there is intruding traffic
  • if the V/S is excessive call "Check V/S" 2000 before target alt
  • do something so that
  • within the last 1000 ft before level off the V/S is not greater than 1500 fpm.
• but do not change anything unless there was actual traffic.

Is it not overly conservative then? No, keep good room when the other plane is not level but manoeuvring against you (in line with ATC clearance).

It wasn't SOP with any of the airlines I flew for, but we all did it anyway - especially in busy TMAs. I still think it's odd that manufacturers never modified their software to perform this function automatically when TCAS came in and TA/RAs became a possibility.

Having to intervene just before the point of ALT capture every time is both a pain and a potential level bust.
But on Airbus; setting 1000'/min and pulling V/S before ALT* is surely much easier? ALT will still capture.

Uplinker

you are aware Airbus has made TCAP but it doesn't come as a standard equipment on A320. You got to perhaps pay for it.