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Airbus ALT*

Old 21st May 2021, 04:50
  #21 (permalink)  

Only half a speed-brake
 
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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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Old 21st May 2021, 05:23
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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.
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Old 21st May 2021, 05:31
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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.
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Old 21st May 2021, 05:37
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Better a heart attack than a 21 gun salute.
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Old 21st May 2021, 05:50
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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...
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Old 21st May 2021, 06:09
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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.
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Old 21st May 2021, 06:12
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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.
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Old 21st May 2021, 08:05
  #28 (permalink)  
 
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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.
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Old 21st May 2021, 08:24
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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...
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Old 21st May 2021, 08:43
  #30 (permalink)  
 
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No! During ALT* leave it alone it's too late.
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.
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Old 21st May 2021, 09:05
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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.
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Old 21st May 2021, 13:19
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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.
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Old 21st May 2021, 15:55
  #33 (permalink)  

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Originally Posted by Check Airman View Post
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.
Never heard of that option before, very grateful for you sharing the wisdom.

Last edited by FlightDetent; 21st May 2021 at 16:09.
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Old 21st May 2021, 16:48
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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.
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Old 21st May 2021, 17:26
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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.


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Old 22nd May 2021, 01:38
  #36 (permalink)  

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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.

Last edited by FlightDetent; 22nd May 2021 at 04:04.
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Old 22nd May 2021, 01:54
  #37 (permalink)  

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Technical specifications, ACAS ver. 7.0


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Old 22nd May 2021, 02:20
  #38 (permalink)  
 
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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?
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Old 22nd May 2021, 03:56
  #39 (permalink)  

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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.
Limit V/S = 4000 fpm (rate) = 3000 ft (to-go distance + separation) / 45 sec (TAU TA) * 60 sec (per minute)
-->> Descending with 4000 fpm gets you TRAFFIC, TRAFFIC audio call 2000 ft before clearance limit.
Assuming pilots reduced the rate and continued with 2000 fpm instead of 4k,
DIST = 1500 feet = 2000 fpm (rate) / 60 sec (per min) * 45 sec (TAU TA)
-->> TA "traffic, traffic" re-activates when 500 feet above the cleared level = 1500 feet from CPA (other traffic). Assuming the other plane is still there.
DIST = 1000 feet = 2000 fpm (rate) / 60 sec (per min) * 30 sec (TAU RA)
-->> a hard RA fires at 1000 before the CPA itself!. Exactly at your 1000 ft separation clearance limit = you need a level bust for it to sound - any sort of altitude capture will prevent it.
---

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).

Last edited by FlightDetent; 22nd May 2021 at 04:26.
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Old 22nd May 2021, 08:28
  #40 (permalink)  
 
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.........in some countries it's SOP to reduce vertical speed to 1000ft/mt within last 2000ft to level off. So the TA are avoided.
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.

Last edited by Uplinker; 22nd May 2021 at 08:45.
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