What do you use for gust correction

[rigpiggy]

I have always used 1/2 constant + all of gust. Out in NL I have had VFE- Vref on the approach (though I added a bit more than 20 kts.)

https://aviationweek.com/business-aviat ... 4bc72ec678


At the airplane gross weight of 117,000 lb., the landing reference speed (Vref) was 140 kt. The crew added 10 kt. to Vref for approach speed and set 150 in the mode control panel (MCP) target airspeed window. The Boeing 737 flight crew training manual (FCTM) procedure is to add half the steady state wind and all the gust up to a maximum of 20 kt. for approach speed. With the existing wind, the appropriate increment was 18 kt., not 10.

[digits_]

Maybe I misunderstand, but why would you add half the speed of a steady wind to the approach speed?

[trey kule]

I was to quick to clarify.

This is about gusty winds.

Say 20G30. The formula is 1/2(20) + (30-20). So you would,subject to operational considerations, add 20 kts.

It should be noted that this formula is intended for heavy, low lift aircraft. If you start adding 15kts to the approach speed of a light aircraft with high lift wings it could cause you some real issues..15kts to a heavy is about a10% increase and has little noticeable effect in landing.
On the other hand, 15 knots of extra speed in a 172 , might make trying to round out and flare in these winds an interesting exercise.

Hope this makes it a bit clearer

[photofly]

Maybe I misunderstand, but why would you add half the speed of a steady wind to the approach speed?

+1.

In the example in the previous post, why add the 1/2(20)?

[digits_]

I was to quick to clarify.

This is about gusty winds.

Say 20G30. The formula is 1/2(20) + (30-20). So you would,subject to operational considerations, add 15kts.

It should be noted that this formula is intended for heavy, low lift aircraft. If you start adding 15kts to the approach speed of a light aircraft with high lift wings it could cause you some real issues..15kts to a heavy is about a10% increase and has little noticeable effect in landing.
On the other hand, 15 knots of extra speed in a 172 , might make trying to round out and flare in these winds an interesting exercise.

Hope this makes it a bit clearer

I understand the calculation. But even in a 737, why would you add 15kts and not 5kts? I could even understand adding the full gust, so 10 kts in this case, but why more?

[rigpiggy]

Because there is no reason to spend the extra time on the approach. That was introduced to me by USAirways years ago, I have used it since. As I said in NL I have gone from 180-110 continuously on approach, I also added V2+15 on departure to give me an extra buffer

[AnonPilot]

Per the Boeing FCTM the wind additive is to provide sufficient maneuvering capability.

The steady wind additive should be bled off in the flair and the gust factor maintained to touchdown. Minimum speed for touchdown is Vref-5

[rigpiggy]

because the extra airspeed will still translate in a lower groundspeed on arrival anyway

[ahramin]

Maybe I misunderstand, but why would you add half the speed of a steady wind to the approach speed?

Easy answer is because that's what the FCTM says. More complicated answer is wind shear. You add the extra speed to deal with any potential wind shear, then bleed it off to reach the threshold without it.

[telex]

[Cessna 180]

i'm not sure about other boeings, but this has been modified to a max of 15 knots over ref with no correction above vref + 5 with a tailwind in the 757/767.

[AnonPilot]

i'm not sure about other boeings, but this has been modified to a max of 15 knots over ref with no correction above vref + 5 with a tailwind in the 757/767.

Still 20 in the classic.

It does make for a very “flat” landing with that much extra speed. The airplane is flown onto the runway.

I think it’s also important to note for people following along but in transport category jets the maneuvering speeds are considered MINIMUM speeds unlike early Cessna training.

[GoHomeLeg]

i'm not sure about other boeings, but this has been modified to a max of 15 knots over ref with no correction above vref + 5 with a tailwind in the 757/767.

The 737NG has been amended to a max of Vref +15kts also with no corrections for tailwinds.

[photofly]

If the winds are steady and strong (rare, I know), do you still add half the headwind component?

[AnonPilot]

If the winds are steady and strong (rare, I know), do you still add half the headwind component?

Yes.

[digits_]

I am intrigued by this.

I get you are following the documents, but does anyone know *why* boeing implemented this procedure?

If Boeing is worried about low speed manoeuvrability, wouldn't it make sense to always add the component, especially when you have a gusty crosswind? Ailerons are usually weaker than the elevator. If you have gusty crosswind, ailerons would benefit more from the effect than an elevator.

Does the landing performance calculation take this into account? (different numbers for autothrottles on/off?)

[photofly]

DP Davies address this briefly in "Handling the Big Jets". His suggestion for what is "usually" added for gusts, and for strong winds isn't quite the same, but he points out "..[N]ot only does the jet aircraft take longer to accelerate [than a piston powered aircraft] but there is no added protection from a substantially reducing stall speed with increasing power."

[telex]

More from HTBJ... (first published in 1967)

An aeroplane is sensitive to changes in wind speed. A sudden loss of
10 knots in wind speed is almost ex!ctly equivalent to a sudden loss of
10 knots in airspeed, with the expected results. This is because the indicated
airspeed decreases much faster than the speed of the aeroplane over the
ground in still air can be increased - and this is more significant in a turbine engined
aeroplane than a piston-engined aeroplane. Not only does the jet
aircraft take longer to accelerate but there is no added protection from a
substantially reducing stall speed with increasing power.
In making an approach with a suspected wind gradient problem, some
protection has to be built-in in advance and this is done by flying the approach
at a higher airspeed. There are various rules of thumb, the most common
of which is to add a half of the reported windspeed to the otherwise chosen
airspeed. This may normally be done up to a maximum increment of
15 knots. Only in exceptionally demanding cases, such as when there is a
need to consider gusts additionally, should this be exceeded.

Gusts

As wind speed increases so does potential gustiness. Gusts are as much the
result of upstanding obstructions causing turbulence downwind across the
runway approach path as the general choppiness at low level on a rough day.
Unlike straight wind shear, which usually results in a lower wind speed at
lower levels, gustiness is pretty random, and can cause disturbances in
speed, height and altitude. The rule of thumb increase in approach speed
due to wind shear will normally be sufficient to look after a proportionate
amount of gustiness, but if gustiness alone is considered to be more important
than shear effect then some extra allowance has to be made.
Moderate increments for shear may be increased to take account of gusts
where, again, the rule is to add half the gust value.
Where shear and gust increments need to be added cumulatively they
may be, up to a maximum of 20 knots above the approach speed chosen for
calm conditions. These two effects together therefore give a maximum
increment of + 15 knots for wind shear plus another 5 knots for gusts.
This speed increment of 20 knots total should provide adequate controllability
for all except really appalling approach conditions. As in many other
absolutely extreme conditions the captain is then authorised to take whatever
decision he considers prudent under the prevailing circumstances.

[co-joe]

The 737NG has been amended to a max of Vref +15kts also with no corrections for tailwinds.

The Max has the same limit of a maximum of Vref + 15 kts (or flap placard speed -5 whichever is lower) instead of Vref+20 which I think the previous models had. I'm not sure why the change. More responsive engines, more slippery airframes, or just more recent data?

In the Max FCTM, Boeing does say that being 50' high = 950' more landing distance FCTM 6.46 whereas being 10 kts fast only adds 170-310' FCTM 6.47 so obviously being a bit fast isn't as big of a factor to them.

The TC Flight Training Manual says a 20 kt tailwind adds 800' to your landing distance on P. 109 which works out to 200'/ 5 kts (which is the rule of thumb lodged in my noodle). I assume that's for the mighty 172 but they don't say. I can't find where they make that assertion wrt Vref but it would make sense that the same rule applies.

[telex]

Because there is no reason to spend the extra time on the approach. That was introduced to me by USAirways years ago, I have used it since. As I said in NL I have gone from 180-110 continuously on approach, I also added V2+15 on departure to give me an extra buffer

If we are still talking about a 737 is V2 + 15 not normal?