Airbus groundspeed mini & unstable approaches
 

Scenario to ponder ...

Flying into airfield "A" on a gusty day. Steady headwind of 25 knots at 1000', which turns to zero passing through about 400' on final. The G/S mini function soaks up the wind shift, aircraft maintains the magenta speed target and aircraft touches down at VAPP without recording an exceedance.

Flying into airfield "B" also on a gusty day, with a 15 kt tailwind at 1000', which rapidly turns into a 10 kt headwind at 400'. Aircraft records a momentary speed fluctuation of up to VAPP +25 before it settles down again. Still touches down at VAPP.

Company does nothing to pilot "A", and kicks ass of pilot "B" because a speed variation of +25 was observed on short final.

Question is then, is one an unstable approach and not the other? Should pilot "B" go around and not pilot "A"? From a P of F point of view, isn't it more dangerous to be flying into the decreasing headwind than the increasing headwind?

in scenario B, you are saying a 180 wind shift, from 15kts tail to 10kts head.....

Really?

Are you talking about a FOQA bust?

"Momentary" is the key. What's the Stabilised Approach max speed parameter? How long was the actual speed above it?

It could be seen that way, but on the other hand, an uncorrected increase in headwind could cause a balloon and a long landing.

The FOQA data in that outfit is obviously not confidential. :cool:

I have had scenario B.

Have a quick look at your IRS groundspeed and if that get you puckering, you know to give it away :)

A competent pilot would handle a or b just fine. Why let the non flying desk guys run your cockpit? I know it has happened now but why? It is really a sad situation when pilots can not do what makes sense because the monitors will bust them.

I was on final At las one day in a 737 and could see my ground speed was low so increased about 20 knots. It fixed the wind change at low altitude so we landed with no problem. It probably would have set off the new alarms however.

It all depends how enlightened and knowledgable the company are with regard to their stabalised approach requirements and their use and understanding of Vapp and Vref.

Exactly, sounds like your company need to learn more about how ground speed-mini works before giving their line guys a hard time.

If they are asking for no speed variations, I guess you can never land in gusty conditions... Where rapid speed changes will occur.

We are not talking about someone maintaining an excess of 25kt, it was a temporary fluctuation, which A, should not of pinged the FQUA and B, when a human looked at the data should of been able to see what triggered the event without calling the crew!

Whatever device your company is using to measure your speed, they should also be able to see the FMS calculated target speed, and the wind speed. So whats the problem? Unless they are unaware that Ground Speed Mini even exists, in which case you should show them your FCOM and make them aware of it.

In our airline, certainly, FDM is more complex than simply looking at whether an event has been triggered. A "High speed below 500 feet" event would result in an analysis of the whole approach. The analysts would, in this case, note that the aircraft met all stabilised approach parameters, and that what triggered the condition was a short term environmental fluctuation - something specifically covered in the ops manual, which says: (or words to that effect)

wind shear is wind shear, it just happens, they should have been able to get that out of the box...

YP, thats exactly my point.

However, many of the Asian carriers for example don't have the right person looking at the extracted frames.

Thus the whole system turns into a punishment tool, something that does NOT improve flight safety at all.

sittingidly: Mm. But the extra energy carried compensates for the reduced thrust, doesn't it? I have little doubt that a pretty careful analysis was carried out before the idea was implemented. And I also have little doubt that if it can be shown that the policy is less safe than alternatives, then the policy would be worked on.

PT6A: Meh. If the company policy is that you should go around (and presumably divert, ultimately) in these circumstances, then do it. Eventually, a manager somewhere will be led to ask why your company's flights don't get in whilst other carriers do, and the policy will change.

Half the steady and all the gust up to + 20 always worked well for me on my simple Boeing :ok:

Yeah maybe I should have been more clear ... I was talking about the fact that groundspeed mini only works because it adds IAS, and then reduces it when the headwind component is reducing on the approach - it's useless when you have a headwind that increases on short final, or a transition from tailwind to headwind.

I do have a regular airfield where this tailwind-to-gusty-headwind situation occurs and we quite often get a big kick up from stabilized VAPP to all of a sudden VAPP+25. I don't think it's a big safety issue as long as the speed is stable at the flare, it's a super long runway anyway. But half the skippers are terrified of it and either go round or write captain's reports on it every time it happens.

I just wish Airbus could have designed something into the mini G/S to cope with this situation.

Ajax,

In order to cope with the variation that you state, GsMini would have to fly an approach below Vls and risk bumping the tail in the flare, obviously something that isn't going to happen.

GsMini is good at coping with a reducing headwind, but physically there is little it can do in the increasing headwind case. In that instance, it isn't any different from any other jet out there...

I suggest that you approach your company and get them to give you definitive guidance on what to do in that situation, if they want you to bin it and try again/divert then do it. When the costs on the route soar, they will analyse it and come up with 'acceptable' risk... Let them define where the line is, don't risk your job finding out!

.....and . . there goes yer bonus! (Very cost effective these companies . . ?)

in the increasing headwind case, thrust is increased to maintain minimum ground speed thus energy is maintained.
its win win

Exactly Natstrack! Some of these airlines are not as enlightened as we would like.

Coping
 

Ajax,

It is not correct to think that G/S Mini cannot "cope" with the situation.

The aim of the game is to prevent a situation where, due to sudden wind variations, the aircraft ends up at a speed that is too low for safe flight or with thrust fully at idle while speed is temporarily above command speed.

In the usual system, command speed on final is a steady value, and autothrust will chase that value.

If with the usual system, there is a sudden increase in headwind component, airspeed will instantaneously be (very) high and (auto)thrust will decrease rapidly, perhaps even to idle, which is undesired if it happens just before the flare (imagine that the wind decreases rapidly now, you will be on speed, but without thrust, engines will be very slow to spool up).

If with the usual system, there is a sudden drop in headwind component, airspeed will instantaneously be (too) low and even though (auto)thrust may increase rapidly, there is a chance of stalling.

The trick of G/S Mini is that it will VARY the COMMAND SPEED, to cut drastic thrust changes.
In case of a head gust, yes, the airspeed will increase, but so will the command speed, so thrust will not be pulled back. If now the gust dies down, you will be on speed, with the required thrust.

You are concerned about the situation of, say, 20 kts tailwind on final approach, changing to 5 kts headwind just prior to the flare (yes, such situations do exist in real life).
If you want to achieve the ideal Vref + 5 over the threshold, the only way is to "fly" the approach is at Vref - 20 (you will get the 25 kts increase just prior to the flare.
However, at Vref - 20, the aircraft does not fly anymore.
The G/S Mini idea to maintain Vref + 5 (minimum safe approach speed) is the best you can do in that situation. The resulting high airspeed over threshold is unavoidable. Remember though, that your groundspeed is not suddenly 25 kts higher! and groundspeed is what onground stopping is all about. Just don't let the extra airspeed prolong your flare, touch down at the required point.

Just think about the LH A-320 accident in Warsawa, a long time ago: they had 25 kts tailwind during approach, tower reported wind was about 10 kts from right front, exactly your scenario. Windshear was (of course) reported, so they (unfortunately) religiously applied the (then current) SOP: increase Vapp to Vref + 20. You can imagine the result over the threshold, there was a fatal overrun.
If G/S Mini had been available (and used) then the speed increase near the threshold would have been much less detrimental to safety.

If you properly think about this system, it will also be clear why you have to input reported steady state wind into the FMGC. The gusts are then taken care of by the variations in commanded approach speed , whereas in "normal systems" the steady approach command speed has to be increased, to make sure that variations around that command speed will stay above minimum safe approach speed (or stall speed, so you will).