Speeds on final approach

[Del Prado]

Noticed a big variation on final speeds today (indicated airspeeds that is, not ground speeds). It wasn't just aircraft late in slowing due to anti icing or higher anticipated speeds due to tankering fuel but gaining and losing 5/10 kts throughout the approach. Just wondered if anyone could offer a definitive explanation?

[Check Airman]

Noticed a big variation on final speeds today (indicated airspeeds that is, not ground speeds). It wasn't just aircraft late in slowing due to anti icing or higher anticipated speeds due to tankering fuel but gaining and losing 5/10 kts throughout the approach. Just wondered if anyone could offer a definitive explanation?

A bit more info would be helpful. For example, the type of plane u fly.

[212man]

I rather get the impression from the content of the post, plus his profile, that DP is talking from an ATC radar controller's perspective. Presumably with enhanced Mode-S functions (able to see IAS)?

[Del Prado]

212man-spot on.

the weather was broken at 2500 and wind 40 degrees off the centreline at 10-20kts.

[Giles Wembley-Hogg]

Hi Del

I can't remember what the weather was like when you were at work, but I think it is fair to assume you are an approach controller at TC, so some possible answers are:

1. ANTI ICE. If it is switched on (engine or wing) half way down the final slope, this can give an increase in speed of a few knots from a previously stable situation.

2. TAIL WIND COMPONENT. If the wind is mainly across the approach with a slight headwind component which becomes a slight tailwind lower down, then an airspeed increase is possible as the flight seeks to maintain the 3 degree path.

3. TURBULENCE/GUSTY WIND. I am not sure what the update rate of your mode S is, but if the ASI needle is moving around quite a bit due to turbulence then I think that this may give the impression of an increase in airspeed if the maximum value happens to be the one that the transponder transmits.

4. INVERSION LAYER. Looking at the time of your posting it looks as if you may have been working in the late evening (afternoon with an eg or D2 is it!?). If there was no cloud, then an inversion layer may have developed which decouples the wind near the surface from that higher up (2000' say). Thus at different levels there may be different tail/headwind components and as the aircraft is bound by Newtonian physics, the airspeed may change as the layer is transitioned.

Not a definitive list. I am sure someone will be able to think up some more reasons. Hope it helps.

G W-H

[Del Prado]

Thanks G W-H,

My first thought was anti-ice but it didn't follow the usual pattern. (if Temp was +4 then icing would be expected from about 6000', so anti-ice would have been on continuously when on ILS, no?)

No tail wind component. it was a pretty stiff breeze. no turbulence reported, wind was pretty consistent from ground to 3000' which is unusual (no veering)
Update rate of the Mode S is between 4 and 6 seconds.

would IAS vary as you enter/leave cloud? It was BKN at 2500' and, I believe, overcast above.

Ps. all your assumptions are correct, except for the early go bit. what's one of those?

[Giles Wembley-Hogg]

D P

It sounds to me as if the cloud at 2.5A might go some way to explaining the pattern you observed. Since icing only occurs when there is visible moisture (in this case, cloud) the engine anti-ice is only used as we enter the cloud. In the case of our 757/767s, the temperature also has to be 10degC or below.

If the cloud was overcast at 6A (for example) then we would run downwind with the anti-ice on. As soon as we drop out of the cloud it would be switched off. As the normal 360/180 or 180/180 heading and speed combo are issued below 6A, the slowing up would all appear normal (anti-ice off). We then slide down the ILS nice and stable at 180kts, hit the top of the BKN layer, anti-ice back on again and the risk of a slight speed increase.

I think most of us would accept this increase of a few knots for a few seconds, since chances are we will be dropping the gear at 2A anyway and want to avoid the use of the speedbrake. If the speed does not drop back as we come out of the cloud and switch the anti-ice off, then I think most people would opt for a slightly earlier gear selection.

The pretty stiff breeze all the way to the ground, although eliminating the backing/veering headwind/tailwind problem, implies to me a degree of gustiness which would cause a fluctuating ASI reading. In such conditions it is sometimes hard to obtain the next flap setting because of speed limitation problems. Again, people may accept a temporary rise in speed (provided the magnitude is small) in the knowlege that as they will quickly pass through the cloud the previous steady state will be reaquired.

Turning to your question about IAS changing on entering cloud. In my experience it does, but I am not a meteorological expert so I cannot explain it with any degree of authority and the following should be taken with a large pinch of NaCl. As an aeroplane enters cloud the temperature changes quickly (albido effect?) as does the air density (moisture content). If the cloud is convective, then the vertical component of the wind will also changes. Whether all these effects are enough to influence the IAS, I couldn't tell you with any certainty or provide you with a reference text, but as a theory I think that they are a good starting point for further thought/discussion/research.

All the best

G W-H

[Del Prado]

Many thanks, that's a really comprehensive response. Sounds like a combination of anti-icing, wind and flying into cloud.

We do allow a bit longer for aircraft to slow down when icing conditions are expected but I'd never have expected the variations in speed we saw yesterday.
It's difficult to provide consistent spacing when some aircraft are catching up on final approach and others aren't, we just have to increase the spacing a bit and also expect more go-arounds, a reduced landing rate and increased delays.

Unfortunately, airline (and NATS) management see a drop in the landing rate and think we're taking it easy!

As an aside, the 757's were great yesterday. They took so long to get back to 160kts it negated the low speed inside 4dme ;)


Anyone else who was flying yesterday got anything to add?

[Intruder]

I don't know why Anti-Ice would change IAS... Whether or not we turn it on (747), target speed is the same. The only difference is engine N1.

Gusty winds are the likely answer here. If the OP is reading IAS, wind gusts will cause significant changes in IAS. The knowledge of wind gusts will also cause the Pilot to adjust target speed upward on final.

I have better things to do than turn off Anti-Ice on final, especially when the winds are not steady. If it is ON at the Approach Checkilst and the ground temp is +4, it will likely stay on. Clearing clouds after the Landing Checklist will not change a thing.

[bluesideup]

Another explanation might be that they were airbusses.

The Airbus has a function known as Groundspeed mini which adjusts the final approach speed. In essence it tries to keep the final approach groundspeed constant. Consequently if the headwind is 10kts stronger at say a 1000' than it is at the threshold, the aircraft's target speed at a 1000' will be 10kts more than it's planned speed at the threshold.

This means that in lumpy(official meteorological term) conditions the airspeed tends to dance about. In particular the speed 'bug' i.e. the target airspeed can move quite rapidly, so if the mode S readout is based on target as opposed to actual airspeed then I should imagine the effect to be quite apparent.

[oldebloke]

DEL don't sweat it,it's gods way of saying pay attention.Who knows what the wind was doing on final.Just don't let the Approach speed get lower than groundspeed readout..Catering for the poss gust loss..and don't chase the airspeed too much with thrust changes,you could end up slow and low power..:ok: