PDA

View Full Version : Question about descent planning in tailwind.


azymik
8th Apr 2009, 22:22
I am copilot of 737-400 and need help for some clarification.
Everyone knows that you have to add some miles for top of descent planning if you descend with some tailwind.
But question is how come that for approach on windshear that is from headwind to tailwind, it will make you come low on path since it is tailwind that should make you glide more shallow like when you plan for descent.
I knew it's different situation but could anybody help me to come up with formula or some explanation that would clarify this it would be really helpful.
Thanks a lot.

Rainboe
8th Apr 2009, 23:23
azymik, it is to do with the inertia of the aeroplane. The aeroplane, being a heavy mass, wants to continue with its same groundspeed. When you start getting affected by a tailwind, imagine a tailwind as fast as the aeroplane's groundspeed, the aeroplane will suddenly have a zero airspeed and will drop out of the sky! An increasing tailwind makes the aeroplane lose airspeed and therefore lift.

No speed restriction, I think you have that the wrong way round. You will descend at an earlier point to benefit from the tailwind in a glide descent. What is the point in maintaining altitude? You will then have to descend with speedbrakes out. In a headwind, you have to cruise longer and descend at a later point to prevent you having to power up in the descent to stop descending far too early- again, why? The aim of the game is to descend so as to arrive at destination with idle power throughout. Therefore, if your descent takes 30 minutes to get down from cruise altitude, the most efficient descent point is where you will be 30 minutes from destination at idle power- further in a tailwind and less far in a headwind.

You are an airline pilot and you have a serious misconception like that, totally confusing our foreign language contributors?

John Citizen
8th Apr 2009, 23:29
I was under the impression that when cruising in tailwind conditions, one would delay descent in order to benefit from the increase groundspeed as an early descent would minimise the tailwind component as you descend.

No. To do this, you would have descend faster (subject to ATC approval) and might need speed brake too. By delaying the descent, you are consuming more fuel the longer you stay in the cruise, compared to idle thrust descent.

Also when cruising in headwind conditions, an early descent would reduce the overall headwind component with altitude and allow you to increase your groundspeed whilst increasing fuel efficiency.


No. As it means you will have to descent with "some thrust on" to prevent getting too low.

The most efficient descent is an idle thrust descent without speedbrake (at an ATC approved speed - 280 knots in Australia).

A descent is basically a glide. Therefore, you will glide further with a tailwind so commence descent earlier. Opposite with a headwind.

azymik
9th Apr 2009, 08:34
Thanks a lot everyone.
Rainboe, since increasing tailwind makes the aeroplane lose airspeed and lift. So, does it mean that if I am descending, and there is increasing tailwind at lower altitude, would it make me eventually come low on path if I use the same airspeed (LVL CHG mode) as I would in zero wind.

Rainboe
9th Apr 2009, 10:35
azymik since increasing tailwind makes the aeroplane lose airspeed and lift. So, does it mean that if I am descending, and there is increasing tailwind at lower altitude, would it make me eventually come low on path if I use the same airspeed (LVL CHG mode) as I would in zero wind.

No it doesn't. The effect of inertia is only instantaneous. A slowly increasing tailwind will gradually increase the groundspeed and make you go higher than you would have been otherwise. But is a rapid wind change hits you from behind, your airspeed will instantly fall and the aeroplane will drop. After many seconds, the aeroplane will increase groundspeed.

Imagine on final approach, a big headwind hits you suddenly. Airspeed will rapidly increase. The aroplane will 'balloon'- go up, groundspeed will start falling. But unless you stabilise with higher power than you had before, you will start to fall below the glidepath. There are 2 reactions- the instantaneous reaction to the wind change, and a separate reaction is the long term delayed effect of the wind change.

eckhard
9th Apr 2009, 11:05
Rainboe has it spot-on but I would like to add:

During a climb to cruise altitude (or descent from cruise altitude), if the wind changes significantly over 1000ft or so, you can get speed variations due to inertia, for the reasons rainboe describes. For example, if descending close to Vmo/Mmo, and you experience an increasing headwind or decreasing tailwind, the speed can exceed the 'barber's pole' and set off the clackers. This is windshear on a larger and more gradual vertical scale than that typically described during approach and landing.

Took off once into some low Sc, tops around 4000', typical for an inversion. The SID involved a turn through 120 degrees and as we broke through the cloud we simultaneously experienced an increasing tailwind (partly due to our turn), an increase in OAT by about 5 degrees and the sun in our eyes! The climb performance suffered momentarily (airspeed dropped by about 5-10 knots). This again illustrated how inertia can affect performance.

Hope this helps,

Eck

azymik
9th Apr 2009, 11:48
Many thanks again.
I got it. So the effect of wind on descending is long-term reaction. But the effect of windshear on approach is sudden or short-term reaction. You guys are awesome.
See you on the sky.

Rainboe
9th Apr 2009, 11:50
Thank you! I have to say I am awesome!

Nobody else does!

212man
9th Apr 2009, 11:59
Further evidence - as if it were needed - as to why I am so selective with my choice of carrier!

Rainboe
9th Apr 2009, 12:08
eckhard, one thing I haven't resolved myself adequately. Picture a jet autopilot engaged doing continual steady 360 degree turns at 200 kts. Then do it in a 200kt jetstream. The aeroplane is stationary at one point and 400 kts 180 degrees later. There are tremendous kinetic energy changes (based on the Earth's surface). Yet the aeroplane steadily and lazily continues turning. Whilst I appreciate you wouldn't expect to have full throttle into wind and idle downwind, I don't see why the fairly rapid kinetic energy changes don't affect the aeroplane in any way. Yet for a sudden wind shear, they do.

CHfour
9th Apr 2009, 12:41
Rainboe,

There are tremendous kinetic energy changes (based on the Earth's surface).

I think the "based on the earth's surface" probably explains it (using the goldfish bowl analogy). Wind shear is like swimming out of one "bowl" into another one with a different velocity, I suppose. It is hard to visualise though.

werbil
9th Apr 2009, 12:48
Rainboe,

What the aircraft is doing relative to the ground has no bearing on an aircraft's performance due to inertia. Inertia affects an aircraft in relation to the difference in wind velocity relative to the aircraft in its present position compared to where it was a moment ago.

Using the 200 knot jetstream example - when the aircraft is traveling perpendicular to the jetstream it is traveling at 200knots sideways relative to the ground yet the rudder will be in exactly the same position as if heading directly into the jetstream. Given the same air pressure, temperature, aircraft weight, power and configuration the aircraft will fly at the same indicated airspeed whether flying into or against the jetstream.

However if you pass through a significant wind sheer where the wind is perpendicular to to the aircrafts flight path the aircraft will yaw and some rudder use will be required to maintain both the heading and track.

It's like the turning downwind myth where if you turn downwind an aircraft will lose airspeed because of inertia. When you turn downwind when you are climbing close to the surface you are generally climbing into faster air - which once you are past perpendicular to the wind direction will result in a tailwind sheer - which will cause a loss in airspeed and performance. Generally when you are turning downwind at low level there is also an obstruction in front of you - the further away from the obstruction you are the faster the wind speed - a tailwind sheer again. Add the fact that many pilots skid or slip because of the illusions that are generated when flying close to the ground in wind will result in more drag and a reduction in airspeed.

That's the way I see it anyway.

eckhard
9th Apr 2009, 13:09
Rainboe, I think for a 'typical jet' rate of turn of 2 degrees per second, the average groundspeed change from 'into wind' to 'downwind' would be 400kts / 90 secs, which equals just over 4kts per second.

That is an average for the 180 degree turn, so the groundspeed would be changing at a slower rate at the begining and end of the turn, and at a faster rate during the middle part of the turn.

One would imagine that this faster rate of change of groundspeed would result in some noticeable effect on the airspeed. I have flown in a 150kt jetstream and have changed heading such that the groundspeed changed by 50kts in 20 seconds or so, but I have to say I have never seen any airspeed change as a result.

We need a physicist to help! Anyone out there?:confused:

Eck

Checkboard
9th Apr 2009, 13:14
The kinetic energy changes of your aircraft circling in a wind is the same from either reference (the earth, or the air mass).

From the position of an observer on the earth, the aircraft goes from zero velocity flying upwind (to the right, say) to 400 knots flying to the left.

An observer in the airmass (sitting in a hot air balloon, say) observes the aircraft flying to the right at 200 knots, and transitioning to flying to the left at 200 knots.

Same change in velocity in both cases, same kinetic energy change, same work done by the lift force of the wings in both cases.

212man
10th Apr 2009, 00:14
Oh no - not the downwind turn myth resurfacing! It's like deja vu all over again....:bored:

werbil
10th Apr 2009, 23:45
Checkboard.

Very well put. :ok: