Ground speed mini

[Sidestick Bob]

Hi there,

I'd just like to know what the general opinion of the "Ground Speed Mini" function is amongst Airbus drivers, particularly on the A320 and A321? Do you find it useful or is it a bit of a double edge sword? Is it reliable?

Thanks

[kellykelpie]

I love it. No need to monitor minimum groundspeed like on the boeing. No need to think. What a great idea.:}

[CaptainProp]

Just remember, if GSmini is "active" with strong head winds its a good idea to configure further out to make sure that you are stable at 1000/500. It takes some time for the system to settle down at your Vapp (Vls+5)......

[wheresthecoffee]

Why was there ever a need to monitor min g/speed on the Boeing?

You're not gonna fall out of the sky cos the g/speed is low, only if the airspeed is low!!

So many aircraft succesfully fly the world without GS mini and without having a problem. It's a useful tool rather than an essential one.

[backofthedrag]

I used to keep an eye on the groundspeed during the approach when flying the DC10. In strong headwinds it felt better with a higher TAS to keep the groundspeed at something above 130 knots. Inertia is presumably relative to earth - you wouldn't want the wind dying a death. And then lo and behold I go on the Airbus and they've spent a few million fitting a device which does what we did all along. Its there for a reason and I flinch every time the other guy goes selected speed, brings the TAS down and sits a few hundred feet above the earth with a GS of 100. We haven't crashed yet so I could be wrong.

[wheresthecoffee]

You won't crash if your g/speed is as low as 50 kts as long as your airspeed is above the stall!! Aerodynamics for beginners chapter 1 folks!!

Even the 'bus couldn't get the thrust up/down quick enough if a very sudden gust knocked the speed up/down 50 kts.

Aircraft do not stay in the air because of g/speed - it's airspeed. That's how every aircraft that doesn't have gs mini does it.

[captainpaddy]

Wheresthecoffee,

You are correct, but only for constant wind speeds. The discussion above relates to rapid changes to headwind component experienced during an approach.

The GS Mini function recognises the difference between the instantaneous wind and the wind reported on the ground and increases the target airspeed to maintain the groundspeed that the aircraft should have at the threshold. This means that if the wind does drop off dramatically, the aircraft will have enough energy to keep flying. So the 'bus most likely would handle large loss of airspeed if the wind at the surface was low (although 50 knots would be pushing it!) and would certainly handle it better than other aircraft. IAS keeps you airborne but GS is very important too!

I personally think GS mini is marvellous. It really comes into play during transient increases in headwind component during the approach. This is an example I was given:

VAPP=140Kts, Surface headwind 10Kts, GS mini=130Kts

Airbus
During approach aircraft experiences a sudden increase in headwind component to 30 knots, the bus sets a new VAPP target speed of 160Kts to maintain GS of 130Kts. This requires an increase in power. A few seconds later the wind shears to zero headwind component and the IAS drops to 130Kts, but since the engines are already at high power the recovery to the original VAPP of 140 Kts is well underway. GS was never below 130Kts at any stage.

Aircraft other than Airbus
Same speeds and conditions.
During approach aircraft experiences a sudden increase in headwind component to 30 knots, power is reduced (possibly to idle) to maintain VAPP of 140kts. GS now 110Kts. A few seconds later the wind shears to zero headwind component and the IAS drops to 110Kts also. If the aircraft doesn't stall immediately, the engines are now at idle and recovery to a safe airspeed may not be possible before the ground arrives. By allowing the GS to reduce on the approach the situation became critical.

Obviously, shears to a tailwind are going to cause problems for all aircraft...

CP

[junior_man]

IF there is windshear around, it could save your butt. Of course, if there was windshear around you wouldn't be flying anything else at ref plus 5 either. GS Mini doesn't cause any problems usually except on those days when there is no windshear but strong winds aloft at low altitudes, in which case you either have to intervene with speed select or you wind up going much faster than everybody else on final.

[Sidestick Bob]

Thanks for all the replies.

The main reason I asked this question is because I myself have had a few "interesting" expereinces on the "bus" where g/s mini was a factor.

A couple of weeks ago I was making an approach to XXX. The wind on the ATIS at TOD was 250/8 and the runway in use was 24. We turned onto the Localiser just as some weather was passing through the airfield. We were both busy on the latter stages of the approach. We started to experience strong turbulence and the surface wind started to increase with gusts. Several wind checks from ATC during the approach gave surface winds of up to 260/32. I guess the average wind was probably around 260/20 (ish). To cut a long story short, I didn't update the surface wind on the PERF page of the FMGC. The result was a very high approach speed (well above VLS). As we crossed the threshold the aeroplane ballooned. We managed to get a landing out of it but it was all a bit hairy!

Moral of the story : update the FMGC PERF APP page for large changes in surface wind!

Has anyone else had any similar exeriences?

[backofthedrag]

Most pilots who dislike the ground speed mini function do so because in strong headwinds the high TAS near touchdown causes a very lively reaction to the flare with balloon, long float etc. However the GS is normal and once the a/c is on the ground normal landing distance applies. The answer is probably to fly it on with little flare and get the best of both worlds.

[wheresthecoffee]

Amazing then that so many aircraft without it manage to ever fly a safe approach!

[captainpaddy]

Ha ha! And I thought I'd been flying safely all these years!!! Oh well...:D

[FlightDetent]

Moral of the story : update the FMGC PERF APP page for large changes in surface wind!


Exactly as manufacturer asks, enter threshold wind as exactly as possible. Available down to 700' feet (data lock).

FD. (the un-real)

[pakeha-boy]

sidestickbob...would agree G/S mini is great....have had sevaral occassions where G/S mini was so high that going flaps full would have exceeded the limitation....would agree with the FMGC change....nothing better than a stabilized app..

[joe]

Hi guys, agree with updating the ATC wind as accurately as possible but am I correct in thinking that this is only possible prior to Activating the approach?.

Rgds

[FlightDetent]

ENTERING THE APPROACH WIND
... This wind is modifiable in descent and apprach and go around phase.
FD.
(the un-real)

[Dream Land]

have had several occasions where G/S mini was so high that going flaps full would have exceeded the limitation....by pakeha-boy
I too have encountered this condition during severe conditions, I thought the books listed the MAX GS mini as 177. Too lazy to look it up now but when I mentioned it to a mate, he told me it couldn't happen, wrong!:confused:

[Dream Land]

Jump seated on the 777 yesterday, looks very similar to the Airbus in many respects, obviously someone appreciates the new technology.:}

[FlightDetent]

With a little reference to the hairy TAP A321 video, wouldn't such a situation suggest selecting flaps 3 (FCOM 3.04.91 p3 - Operations in windshear or downburst conditions / during approach)?

FD (the un-real)
... takes cover for the flak ...

[cornwallis]

No one has posted that if you are unhappy with managed speed you can revert to selected!I have seen the managed target be above flap limiting speed,so you Select a speed below,extend landing flap and then revert to managed.A lot of the problems I have seen are due to the aircraft actually being a couple of tonnes heavier than the loadsheet which reduces the vapp/vls to less than 5kts.As an aside the Lufthansa 320 accident in Warsaw? had the wrong wind entered in the fmgc.

[FlapsOne]

A lot of the problems I have seen are due to the aircraft actually being a couple of tonnes heavier than the loadsheet which reduces the vapp/vls to less than 5kts.

A couple of tonnes!!!!!!!!!!!!!!!!!!!!!!

Out of interest I have just read the LH accident report you refer to and there is no mention of incorrect wind entries in the fmgc. it does however refer to reported, but largely ignored, windshear, plus tailwinds on finals caused by a rapidly passing weather front.

[757operator]

A couple of tonnes is nothing, just the difference between the standard weights on the loadsheet and what's actually in the hold.

Allowable baggage weight + cheat slightly = 22Kg, less 13Kg per loadsheet bag = 9Kg, x 220 bags on a charter A321 = 2 tonnes. QED.

[FlapsOne]

Yeah but the pax std weights more than compensate for that........................................unless you're over the atlantic!

[pakeha-boy]

Dreamland......flaps full speed is 177ts for sure,and as someone has suggested ,to revert to selected speed then go back managed is a practice many use,including myself.....and there is I beleive a limit on the GS mini and how far it will go(will try to find that out)....the problem I see is during very gusty strong winds is that even with the GS mini at its max....the resulting fluctuating airspeed puts you so close to the flaps full speed limitation(177kts) resulting,obviously, in the overspeed horn.
These occasions do not happen very often,but they certainly get your attention.In the app phase the managed speed target is the mini ground speed target computed by the flight guidance(FG)part of the FMGS...

[320operator]

1.22.30 Autothrust

The lowest speed target is limited to VAPP and its upper limit is VFE of next configuration in CONF 1, 2, 3 and VFE - 5 in CONF FULL.


System protects itself quite nicely....no need to stay switching unnecessarily between selected/managed unless you have terribly strong headwinds

[cornwallis]

I have frequently seen the top of vls 2 or 3 knots higher than it should be.In the cruise look at green dot and do the maths in the QRH to work out your gw.Typically in IT config you will always be two tonnes heavier than the loadsheet!I then use this figure to decide whether to step or not.Vls comes from the facs( a of a) info and is more reflective of the true weight

[pakeha-boy]

Cornwallis...that is true,and one way to do.....if you also go to the AIDS page,pull up...Alpha Parameters(second line).it will bring up a menu...gives you 4 blocks in which to put a code...punch in (type,line select,1L)GWFL or GW,it will do the same thing for you....I use this all the time to determine Max FL,for A/C weight....

320 DRIVER,....Thanks for the info

[320operator]

pleeeeeeease don't mix me up with the driver.....I'm the operator!!:D :D

on a different note...... using AIDS to determine MAX FL sounds a bit too much!!....... use the PROG page it is more than enough. Even with a difference of a couple of tonnes you are not going to fall out of the sky if you climb to what is displayed in the PROG page.

[pakeha-boy]

320operator...sorry mate!!
Would agree with the prog page and use that all the time....I only bring it up as it is another "Tool" that Airbus allows us to use and play with...comes in handy and makes you a little more aware of the aircrafts capabilities,what you can and cant do......5-6hr flights...idle fingers

[oldebloke]

Where's the coffee,SOMETHING else that hasn't been passed down..
In the good old days of windshear on final ,crews used to select one INS to Grounspeed and didn't let the grounspeed decease below VAPP,regardless of what Airspeed was indicating at that moment(catered for the gusts,and the sudden loss of):O
cheers..