aerotech777 From United States of America, joined Aug 2009, 48 posts, RR: 0 Posted (1 year 1 month 21 hours ago) and read 1927 times:
Hi,
a) Why you add wind additives such headwind and gust to Vref?
b) Why you don’t subtract/add wind additives such tailwind and crosswind to Vref?
It seems Boeing recommends bleeding off headwind additives approaching touchdown.
c) How you can bleed off wind headwind additives approaching touchdown?
d) How you can bleed off other wind additives/subtractives (tailwind, crosswind)?
I mean by subtractive the opposite of additive (if this word exists in English, too lazy to check it in the dictionary)
These questions concern all aircraft manufacturers (Boeing, Airbus, Bombardier, Embraer...etc)
PapaChuck From United States of America, joined Aug 2010, 135 posts, RR: 0 Reply 1, posted (1 year 1 month 19 hours ago) and read 1891 times:
I'll take a stab. The professionals around here can provide the specifics, but here it is in layman's terms so I can understand it as well.
a) Gust factors are included to ensure that a sudden drop in airspeed won't leave you too low and too slow. If the conditions are known to be gusty, there's a significant chance you could suddenly lose airspeed, so you fly the approach a little faster to compensate. If you carry an extra five knots, it's no big deal if you suddenly lose five knots due to a gust or windshear.
For known headwinds, consider this: Say your normal Vref is 130 knots and you typically descend at 700 fpm to stay on glideslope in calm conditions. Now, add a 30 knot headwind. Your groundspeed is now 100 knots at Vref. If you descend at your usual 700 fpm, you'll end up too low and short of the runway. You can either add power and shallow out your descent, or add some additional airspeed to even things out a bit.
b) It's always a bad idea to fly below Vref. Subtracting from Vref will put you dangerously close to a stall, and short final is the last place you want to worry about stalling the airplane. There's no room to recover.
A direct crosswind won't affect your groundspeed. You have to crab in to the wind to keep it on the centerline. However, the transition from air to pavement can get a little tricky in a crosswind, so you carry a little extra airspeed as insurance.
c) The simple answer is to reduce power as you start your flare over the threshold. With the added drag of the flaps and landing gear, it's easy to bleed off airspeed by chopping the power. However, if you're carrying a little more airspeed in gusty conditions, it's better to plant the airplane on the pavement a few knots fast rather than bleed it off in the flare. Just land it and get on the brakes.
d) Again, you never want to intentionally fly an approach below your recommended approach speed. Even if you have a tailwind, you need to maintain enough airspeed to safely keep the airplane in the air. Just plan on landing at a faster groundspeed and having a longer landing roll. This is the main reason you always want to land with a headwind if you can.
We add 5 knots to the Vref if wind is less than 10 knots. Then you are safe for smaller changes in wind speed and direction as you never want to fly slower than Vref.
Gusts are quite important. Imagine Vref 140 with an approach speed of 145 and gusts 10 knots, you fly 155. Imagine the gust suddenly stops and you only fly 145 knots, then you are suddenly 5 knots below the Vref, which is a no go... So add the gusts and keep them until touchdown.
Quoting aerotech777 (Thread starter): Why you don’t subtract/add wind additives such tailwind and crosswind to Vref?
Crosswind doesn't matter. If 10 or 20 knots crosswind doesn't change a lot aerodynamically over the wing itself.
And if tailwind is decreasing, then it is aerodynamically safer.
Quoting aerotech777 (Thread starter): How you can bleed off wind headwind additives approaching touchdown?
If you fly faster, you need more power, so start reducing power slowly when approaching the runway and let the speed bleed off a bit. Most of the times you still touchdown faster than Vref as the mass of the airplane makes it difficult to reduce speed in a short time.
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