Stabilised Approach Speed Tolerance

[Hudson]

Among other things, the Boeing 737 (Classic) Flight Crew Training Manual recommends that at (1000 ft IMC and 500 ft VMC) final stabilisation height, the airspeed should not be more than VREF +20 IAS and not less than VREF.

I can understand the logic of "not less than VREF", but a IAS of 20 knots (when it is not required operationally) above VREF at 500 ft is really stretching the friendship especially if the runway is wet.

I realise that VREF +20 is the certified upper limit where an unacceptable risk of exceeding the certified landing distance for the weight is possible, but if the +20 knots is the result of incompetent handling or a unexpected wind gust, then no way would I have thought this speed would meet the spirit behind a calm day stable approach criteria.

There are operators who publish their stabilised approach airspeed criteria as no more than 10 knots above the selected approach speed, or less than 5 knots above the selected approach speed. That sounds reasonable to me. In other words, in no wind conditions, a speed of VREF say at 500 ft would be within the tolerance and no support call-out is required.

The FCTM states that the recommendations published (in this case for speed tolerances) are consistent with criteria developed by the Flight Safety Foundation. That maybe so, but the +20 knots (unless operationally required) part seems a a bit too liberal for my liking.

Appreciate if readers could give any examples of other B737 operator's definition of stabilised approach speed criteria to see how it compares with the Boeing recommendations. And if different to the FCTM figures, why?

[Wizofoz]

Vapp is reccomended as Vref+ additives for wind (1/2 steady headwing +all gust factor) up to a maximum of Vref+20.

Sops where I have worked then give a stabilised approach criteria of Vapp +10/-5. Theoretically, therefore, if you've used the whole 20kts addititve (say a wind of 20 gusting 30) you are still within stabilised approac criteria at Vapp+10 =Vref +30.

[NigelOnDraft]

I think you're missing a farily fundamental point...

You still have 1000'/500' remaining to sort out this 20K problem... Your "gates" as you get sub 100' should not be as wide as this.

Since most types will use a "theshold" or "final approach" speed of, say Vref+5, you are looking at 15K to lose over 1000'/500' - hardly a problem, and in "stabilised approach" criteria will not need "idle" thrust to lose, just a little less than normal approach power.

The approach can therefore be considered stabilised...

All your concerns about wet runways etc are a completely different issue.

At BA we have the 20K at 500' limit, and completely different limits at the threshold...

[Hudson]

Nigel OD. I am not quite sure what you mean by saying that I have missed the point. There is no point at all in having any stabilised approach criteria if you are going to ignore it to suit yourself. This is how accidents occur.

If the aircraft is not stabilised by 500 ft VMC or 1000 ft IMC within the parameters recommended by, in this case, the B737 FCTM, then good airmanship was suggest that you should go-around and try to do better next time. You can also leave it late and go-around at the flare if you like although you might have some explaining to do to your PR people.

Fine if your company wishes to publish a lower stabilisation height such as 200 ft - but I would prefer to stick to the manufacturer's recommendation for both safety and legal reasons.

As I mentioned in the opening post, I am aware that wind and gust additives may require a maximum of 20 knots to be added to VREF. But that was not my point. My point is that accepting +20 knots above VREF as a matter of normal no wind landing procedure is excessive especially if the runway length is marginal and it happens to be wet, or at night when distance remaining is sometimes difficult to judge accurately

[Right Way Up]

Hudson,
I'm with NoD on this one. Your example seems to work on the basis that being VRef + 20 (calm day) will be kept to the threshold. This obviously is poor airmanshop!NoD is saying that you can be stable at 500 feet at Vref +20 and power up, satisfy the FCTM, and still be above the threshold at Vref + 5.

[despegue]

Let us not forget that the B737 is in my opinion quite speed unstable.
Therefore, a Vref+20 at 500' is in my book acceptable, as long as the speed is reduced (without having to go to less than 45%N1) to be on speed over the threshold.
But obviously, it shouldn't happen on a calm day...:\

[LEM]

Good question, Hudson, as usual!

I see it this way:

The Pilot Monitoring (as it is called today;) ) will call "SPEED!" for an error of +10 or -5 knots, in all phases of flight.

Thus if we have determined we have to use a 20kts correction because of wind, the PM will stay silent until Vref+30.

It would be absurd to think that you should goaround at Vref+21 if you are aiming at Vref+20.

As the Training Manual states, we should keep the gust correction all the way to touch down, while bleeding off the steady correction during the flare.

So, until the beginning of the flare, on a calm day I will consider the approach to be stable until Vref +15 (+5 knots standard minimum additive, plus 10 knts tolerance.)

Clear as mud? Maybe not! ;)

LEM

[HSWL]

One defintion of a stable approach is where the aircraft maintains a predetermined speed and configuration. If for any reason you are therefore 20 knots above Ref at 500/1000 and you are attempting to maintain VREF+5, then simply you do not meet the requirements of a stable approach. In attempting to reduce your approach speed by any means in order to get back to VREF +5 before the threshold, then again, you are not stable. A go-around is required if not stable at 500 ft VMC or 1000 ft IMC.

Going back again to the purpose of the original post (sorry if I am repeating myself) which is that accepting a speed range of +20 to VREF itself seems an awfully wide range of speeds to be defined as stable at say 500 ft. Calm conditions, remember!

[Menen]

So often in aviation we forget the lessons of the past, particularly when those in the industry are relatively new to the aviation fraternity.

A stabilised approach is supposed to be just that. Not bleeding speed in the last 500 feet. Not selecting flap late. Not having to use large thrust/power changes. Not having to make large lateral corrections ( a good guide - stay within the extended edges of the runway for a narrow body).

What stupidity to be unstablized in the most critical area of a sector. For goodness sake, review some past histories and ask yourselves, 'WHY NOT BE STABILISED"......or can't we fly within exceedance limits.

Perhaps we should do glide approaches and extend the gear and flap in the last 100ft. Saves fuel and not a problem to the aces yet to have their comeuppance.

The quote I heard recently............."a stabilised approach is too restricting. If you can't make all the adjustments needed you shouldn't be flying".

Now there's a professional safety conscious captain, yes?

[cjam]

HSWL. I suppose that is one definition of a stable approach. I reckon an approach that is on profile and has the speed uniformly bleed from Vref +20 at 500ft back to Vref at the TCH is stable. It results in a more gradual power reduction and pitch change,, it may even be more stable than holding more power to bring it in at Vref, and then bringing the power back more quickly.
If you are holding 15kts for gusts you still have to get rid of that energy somewhere. Where do you lose it? On the ground or drifting around just above it? I would genuinely like to know the answer to that question because I am willing to change the way I do things if I see a better way. Cheers.

[411A]

<<A stabilised approach is supposed to be just that. Not bleeding speed in the last 500 feet. Not selecting flap late. Not having to use large thrust/power changes.>>

Doesn't always work out that way, Menen, at least on some types.
Lets consider the Lockheed TriStar as an example.
All coupled up in the approach/land mode, with auto thrust engaged.
I you watch carefully of how the autothrust does it, you might find the following, during gusty wind conditions...

1000 agl (with landing flap selected)....Vref +35
500 agl......Vref +30
200 agl.....Vref + 25
Crossing the threshold....Vref +20.
Touchdown....Vref+15

The Lockheed design engineers went to a rather big effort to design the system for the very best possible results.

Who are you to say that this idea is not correct?
And further, if the pilot hand flies the approach using the very same technique, why should it be 'dangerous' or unacceptable?

To adhere to Vref+20 rigidly at 1000 agl (or even 500 agl) when the aircraft autothrust does it quite differently, in very gusty wind conditions, is very foolish, in my opinion.

[NigelOnDraft]

Hudson...

My point is that Vref + 20K (which is prob +15K on normal Vapp, but maybe nearer desired Vapp in gusty conditions) @ 500' is "as near as dammit stable".

The idea of the stabilised criteria is that you are configured, on profile, and have some power set in the latter stages for quick engine response if a GA is needed - which in turn means IAS not to far in excess of Vapp...

There has to be a "tolerance", and both the Boeing Manual you quote, and at BA, we use the +20K. It is designed to stop people continuing from, say, +40K, and neededing idle all the way down, and probably still fast...

That is completely different to saying the +20K is acceptable at the threshold. It is not, and we use +15K for a limiting runway.

If you are unable to manage a +15K (or less) reduction over the nearly 2 miles from 500', and are going to rigidly apply those criteria, I can only assume you are well practiced at Go Arounds :) Any sort of gusty weather, and you will often see the IAS above Vref+20K for periods at or below 500'...

Final point, don't worry about whether you agree with me or not. That the Boeing Manual and BA and seemingly others use this criteria seem to indicate it is well established, and I will go for their judgement...

[Right Way Up]

Menen,
<<and ask yourselves, 'WHY NOT BE STABILISED"......or can't we fly within exceedance limits.>>

What we are saying conforms to the FCTM, being at Vref + 20 at 500ft with power up is stable as per Mr Boeing. By bringing in your own ideas about stabilisation you could possibly cause dispute in the flight deck. Personally I always aim to be stabilised by 1000 ft AGL at the latest which gives me a 500 ft buffer to cover for the unexpected.

[Rananim]

Whatever happened to Captain's discretion?If in the opinion of the Captain,the approach and landing can be continued taking into account all relevant factors(incl very importantly PF's abilities) then sobeit.The 1000' IMC and 500' VMC rules are there and are good rules of thumb but they should never supercede a Captains judgement.An airline that utilizes approach monitoring equipment to ensure that its pilots are held to these rules of thumb treads a dangerous path.We live in a world where todays pilots sometimes sweat at the thought of taking out all the automatics at 10000'...we dont want to exacerbate things by taking away his/her right to cognitive decision-making.Condition a pilot too often to rule-based thinking and pretty soon thats all you will get..an emasculated automaton.

[Centaurus]

Rananim. In the immortal words of "Yes Minister" Congratulations on a very courageous statement indeed.

Captains discretion has probably killed more people than going around from an unstable approach. The dirty dart at the deck is preceded usually by "I can make this, let's give it a go". A recipe for potential disaster, unfortunately. That is why stable approach criteria are designed - to stop the cowboys.

[cjam]

Centaurus....I get your point but I don't think it is that 'cut and dried'. I think that the safest operation would exist somewhere in between your ethos and that of Rananims'.
Thats the tricky bit, if it was that simple , if we could just follow certain rules and flight safety was achieved 100% of the time then we would have been doing it for decades. There is an undeniable element of uncertanty and change in any approach and if the pilot is experienced and proficient they can normally absorb these variables without too much planning or conscious thought by simply reacting.
I agree that 'captains discretion has killed more people than going around from an unstable approach' but in all the flight-deck transcripts I have read over the years I think I have only ever seen one that had something that could be loosly translated as "I can make this...lets give it a go"...in fact none resembled "lets give it a go", one is similar to "I can make this".
Cheers

[FullWings]

Good post cjam.

I think the crux of the matter is: "Is the approach under control?".

If you're at idle thrust, 1.5 dots high and hanging on the speedbrake at 500'/1000'/whatever, then probably not and throwing it away would be best.

If the speed is not quite where you would like like it to be but corrections are being made and are having an effect then continuing is a possibility. Obviously, if you're outside your company specific limits at your 'gate', then carrying on is not a good idea. On the other hand, if it's just turbulence/wind gradient/thermal motion causing a momentary departure from normal parameters, I don't see why that should cause undue alarm, as long as you're doing something about it.

I'm glad that there has been a 'sea change' in culture from "Yee-Haw!" sideslipping glide-ins with top-ruddered final turns to the 1000'/500' ARTE stabilised approaches we all aim for today. At the same time, we shouldn't let it become a 'numbers game' and should try and remember some of the reasons why changing to stable approaches was such a good idea:

* Power up before landing = engines in the rapid acceleration range, so able to cope with unplanned G/A, etc.

* Excess speed over threshold + long flare = deep landing & possible overrun.

* Stable airspeed, pitch & power make it easier to detect the onset of windshear, i.e. one or more of these parameters is behaving unusually.

* Both guys are 'in the loop' with the aircraft operation, as opposed to the 'fighter pilot' style of fast (rushed) approaches.