Stabilised approach in gusty conditions / with wind shear

[Nicholas49]

Hello

On a flight back to Luton last weekend, the approach was bumpy because, in the words of one of the pilots, it was 'blowing an absolute gale'.

I know that airlines insist pilots adhere to stabilised approach criteria. I understand the reasoning for this. I understand that if the approach has not been stabilised by a certain altitude, or becomes unstable below that altitude, a go-around is mandatory.

But what I wanted to ask is this: if you are flying an approach in gusty winds, how do you still 'stabilise' the approach? Surely some of the parameters for a stabilised approach do not remain constant in such weather conditions? Or is that not correct? To the non-pro sitting in the cabin being thrown around, I was thinking: 'is this 'stable'? It must be!

What criteria do you use to establish whether the approach is still stable? Can you 'measure' the turbulence?

Also, is there any flexibility to the stable approach criteria when flying in gusty conditions? I imagine that it might be possible for the autopilot to re-stabilise the approach after a sudden gust of wind? Or is this verboten?

I know some aircraft have a 'Windshear' aural call-out. Am I correct in thinking that if this alarm is triggered, the approach has become destabilised and a go-around will be performed?

Sorry, quite a few questions and apologies for mixing up any terms due to misunderstanding. But I would be really grateful if someone could explain a little more how you handle such conditions.

Thanks
Nick

[CharlieRomeo]

stable approach criteria in our airline means:

approach speed Vref + or less 30kts
a/c within +/- 150ft of specified crossing altitudes
a/c in final landing configuration (i.e gear down and final setting of flap set)
landing checks complete to last item (which is the automatics to come out)
a/c must be in a clear position to land without exceeding 25degrees angle of bank

so when the aural 500 call comes, the reply from Pilot Monitoring is stabilised.

Hope this helps, I know it will vary from airline to airline but thats just an example of what our criteria is to call an approach stable.

[Denti]

In my current company any approach has to be stable latest at 1000ft, earliest time a "land" decision can be made is at 500ft. To be stable we need to be in landing configuration, on speed (between Vref and Vref +10 + wind correction, on the boeing that is between Vref and Vref +30), apropriate sinkrate (less than 1000fpm), engines spooled up (apropriate for aircraft type and configuration, on the boeing a good minimum value is 40% N1) and landing checklist completed. The only time we can lower our gate for a stable approach to 500ft is on a circling approach with visual contact to the runway.

[411A]

In our small charter company (L1011 aircraft) we do not publish stable approach criteria...this is left up to the operating Commander, with regard to the specific conditions found, on approach.
Works well, because...we have very experienced folks at the helm.

[Miserlou]

It can also be beneficial to hand fly in very bumpy conditions. My technique is to not be as aggressive to maintain the glideslope as the autopilot is.
Think like a bird.

[vikena]

Yeah that's right.

Birds think about stabilized approaches all the time.

V

[Checkboard]

if you are flying an approach in gusty winds, how do you still 'stabilise' the approach? Surely some of the parameters for a stabilised approach do not remain constant in such weather conditions?

Once the aircraft is configured and on speed, it is "stable" for the sake of the landing, however some parameters do indeed move about in gusty conditions.

What criteria do you use to establish whether the approach is still stable? Can you 'measure' the turbulence?

The speed is the criteria which moves about the most. The tower passes a ground wind which includes the strength of the gusts near the runway - and this information is used to adjust the approach speed (to increase the margin).

Also, is there any flexibility to the stable approach criteria when flying in gusty conditions? I imagine that it might be possible for the autopilot to re-stabilise the approach after a sudden gust of wind? Or is this verboten?
Yes, there is flexibility - in my airline it is a simple comment that speed excursions outside of the stable criteria may be made for gusts (no actual number is given) - so if you are averaging the speed about the calculated approach speed (adjusted as above) then you are still considered "stable".

I know some aircraft have a 'Windshear' aural call-out. Am I correct in thinking that if this alarm is triggered, the approach has become destabilised and a go-around will be performed?
Usually, but not necessarily. My airline allows you to continue for a windshear warning, provided that course of action has been specifically briefed before the approach, and the warning isn't associated with frontal weather. You might consider this in stream weather at an airport known for particularly turbulent approaches, depending on the strength of the shear. The windshear warning was developed to protect against microbursts, so in the absence of the storms which produce microbursts then the system may produce a warning in conditions which (while rough) are not threatening.

[de facto]

n our small charter company (L1011 aircraft) we do not publish stable approach criteria...this is left up to the operating Commander, with regard to the specific conditions found, on approach.
Works well, because...we have very experienced folks at the helm.

If this post werent coming from you 411A, Id be worried...:eek:

approach speed Vref + or less 30kts

Charlie romeo,
What type are you flying?

[Northbeach]

Nicholas49,

You really do come up with some of the better questions! With this post I will try to take on just one.

“Stabilized approach” is a phrase describing a set of parameters that must be met concerning the state of the aircraft during an approach to land while encountering a constantly changing set of environmental variables. Do not confuse the literal definition of the word(s) stable/stabilized with what you as the passenger may personally experience, or expect to feel, on an approach to landing.

The key to your question may be understanding what “stabilized by a certain altitude” means. In our airline, to be stable on approach, is to have the aircraft’s flaps & slats correctly deployed, the landing gear down, the engines developing the correct amount of power, being on the target airspeed (and within the acceptable slight variations) and being on the correct vertical (and lateral) descent path (on the glideslope & localizer or PAPI/VASI) at a given distance and altitude from the landing runway. It has nothing to do with our passengers having a smooth –stable- ride (that experience is always important but not always possible).

So if I have the jet on airspeed and properly configured at the various target distances from the landing airport I am “stabilized”, you however (as you put it so very well) may not believe for one second that you are experiencing anything remotely "stable".


To the non-pro sitting in the cabin being thrown around, I was thinking: 'is this 'stable'?
Sitting in the back there is nothing “stable” (literal definition of the word) about your condition; you are being pulled against your seat belt with every movement of the jet as it reacts to the turbulence outside and the pilot’s inputs on the controls. So no, you are not at all factually “stable”, yet if the jet is within the configuration/airspeed/power/altitude/rate of descent the jet is said to be “stable” for the approach and can continue for landing.

Hope that helps.

Northbeach

[411A]

It is interesting to note that some aircraft, with extremely accurate autoflight/autothrust systems, and those systems engaged properly, it is indeed possible to notice approach speeds of Vref+40 at 500 agl, decreasing to Vref+20 at the runway threshold, under rather heavy turbulence conditions on approach for landing.
IE: the aircraft systems are designed to do just that.

An example would be the L1011.

[The Range]

Stabilized app. in a DC-9:

500 ft. agl. over the threshold
140 kts.
20 degrees nose down
V/S of -3500 fpm

[DERG]

The ultimate how NOT to wait too long video:

YouTube - Lufthansa Airbus A320 wingstrike at Hamburg during Emma (http://www.youtube.com/watch?v=zGBfA3saZ5Y)

Sleep well!

[Airbus_a321]

@411A
In our small charter company (L1011 aircraft) we do not publish stable approach criteria...this is left up to the operating Commander, with regard to the specific conditions found, on approach.
Works well, because...we have very experienced folks at the helm. :ok:
You should be lucky to fly in such a company. I agree and it should be like this: it is"left to the good experienced Commander"...and not to some Stupid Operating Procedures builder from the office, to determine what is stabilized and what is not.

[Northbeach]

This is either a total wind up or somebody's soultion for avoiding ground fire while trying to get into some global "paridise"........

Stabilized app. in a DC-9:

500 ft. agl. over the threshold
140 kts.
20 degrees nose down
V/S of -3500 fpm

[The Range]

Well, make it 300ft. over the treshold.

[Checkboard]

I agree and it should be like this: it is"left to the good experienced Commander"...
YOU might be a good one - but can you say that EVERYONE in your company has that skillset?

Establishing and concentrating on stabilised approach criteria, backed up with flight data monitoring, has been the biggest safety jump in the last 10 years.

[GlueBall]

...it is left entirely at the commander's discretion if the final approach is to be continued. Commanders must decide, commensurate with their experience level on type, whether to continue or not.

SOPs do not address all situations. In gusty conditions, a transient exceedance of stabilized approach criteria would not necessarily dictate an automatic, instant missed approach. It depends on altitude, attitude, speed, control-surface displacements. [Example: If continuous full aileron input is necessary just to maintain wings level on short final, then there would be no margin for roll control and a missed approach would be prudent]. It's altogether a judgement call.

[de facto]

Glueball,

I agree with you to some extent.
However I believe that SOP stabilized criteria are also set to avoid a tragic outcome where the captain mindset is focused on landing the aircraft and not necessarily in a proper position to carry on in the safest way.
(experienced doesnt necessarily mean you arent a cowboy, complicent or dangerous).

SOPs are there too to allow the PNF who has a possibly bigger picture (lower workload to call for an unstable approach and a go around.
It works i believe towards safer flightdeck especially in airlines where the CRM is low.

Of course it is then up to the captain to continue or not but if it ends wrong(long landings,overrun,blown tyre or worse), the captain will have to answer a much more serious interview....

[Nicholas49]

Thank you all for the very informative replies. I have a clearer picture now of what 'stabilised' means.

Northbeach - thank you! Just to be clear, I did of course think the approach on my flight was stable, as I'm sure the pilots would not have fancied a meeting with the Chief Pilot if it weren't!

While we are on topic, can I ask a question about this (http://www.youtube.com/watch?v=rQfESkzHPtM) video? (You will need to mute the audio so as not to get annoyed by the silly commentary) I know it is impossible to say for certain without being on the flight deck, but do you think the go-around here was most likely due to the wind shear that lifts the left wing up on touch-down, or the fact the final touch-down occurred too far down the runway?

Nick

[Checkboard]

The touch down looks like it was just after the 400m touch down zone markers, which is about perfect, and the wings didn't move enough to bother anyone - I would say something on the runway, perhaps a vehicle or aircraft rolled over a stop line? Who knows?

[Northbeach]

Nicholas49,

I watched the video several times. Here are some observations; at 00:23 the female narrator remarks about the “bit of Turbulence”, at 00:36 some sort of verbal exclamation (possible reaction to more turbulence), at 00:56 the painted surface of the fixed distance runway markings appear, the 1,000’ (if I have correctly identified it) marker passes by at 01:01.

Based on what was said it appears there was some turbulence on approach. From where the jet appears to touch down, the first time, the landing was certainly within the touchdown zone. They bounced once got airborne a second time and landed again. The spoilers/lift-dump/speedbrakes did not appear to deploy from the upper surface of the wing after the first or second contract with the runway.

Nothing seems terribly out of sync, I do not know how long the runway is or how heavy the jet. I am unable to provide the definitive answer for the go-around, only conjecture. Perhaps the crew was worried after the first bounce that it would become dynamically unstable with further bounces. One or both of the crewmember was uncomfortable with what they saw happening. Or perhaps the air traffic controller ordered them to go-around because of somebody else’s actions or inactions on or near the landing runway.

[Landshark737]

Could somebody please explain what the acronym 1500U means.

Thanks in advance

[Crazy Voyager]

Huh? As in what? I can't fin it mentioned in the thread at all.

The first thing that pops up in my head is reporting of RVR in METARs though, 1500U would mean 1500 meters with a trend going up. Is that what you're looking for?

[FlightPathOBN]

Look up FOQUA....

[Gulfstreamaviator]

Unless the FD gave a reason for the touch and go, the approach looked ok, no excessive wing or spoiler deflections, TDZ respected.

So suggest Goat on Runway.

[Doors to Automatic]

I suspect that the aircraft floated after the first bounce and the go around call was made just before the second touchdown, maybe because the PF wasn't sure touchdown would be in the TDZ.

By the time the aircraft touched down the TOGA buttons had been pressed and it was too late to stay on the ground. Just a theory having watched the video several times.

[Mikehotel152]

maybe because the PF wasn't sure touchdown would be in the TDZ

That's unlikely to be the reason - unless it was a very short runway indeed.

[Pull what]

400m touch down zone markers 400m, always thought they were at 300m- 1000 feet

Establishing and concentrating on stabilised approach criteria, backed up with flight data monitoring, has been the biggest safety jump in the last 10 years.

Fully agree-closest i have come to a serious accuident is when a Captain tried to recover a very unstable approach below 500 feet- i would say we missed the edge lights by less that 6 feet at night in rain. I am ashamed to say I just sat there

[safetypee]

Re # 28; See section 3.5 Touchdown zone markers.

Landing long: Why does it happen? (www.nlr-atsi.nl/eCache/ATS/16/764.pdf)