Wind Altitude Trade

[ice2x01]

Hello,

I would appreciate it if someone could clarify the following subject for me:

As far as the wind altitude trade is concerned: The way I understand it is that it is basically the trade of optimum altitude for best specefic range for a new altitude which may have winds that will increase range moreso than will decrease specefic range (fuel mileage essentially). Is this correct?

What is the Wind Altitude Trade Factor? I understand there is a table you look at to determine whether or not the new Altitude Wind gains will be better for range or not and you determine this by using the factor from this table, but can someone please explain how it is used?



In the following example:

Cruising speed LRC at FL 330 (Optimum Altitude)
Gross weight 42 Tons (Step Climb Weight)
Head wind component 100 Kts. (Jet Stream)

If you have a wind reports (from another aircraft) informing you that the head wind component is 110 Kts. at FL 370 and only 50 Kts. at FL 290 what would you do?

Wind factor for FL 370 0
Wind factor for FL 330 10
Wind factor for FL 290 38

Flying at optimum altitude and optimum weight is the target every pilot wants to achieve. In the above situation, you are ready for step climb to FL 370 (2000 feet a bove optimum) How is FL370 2000ft above optimum if optimum is FL330? Or do they mean that FL350 will become optimum by time the aircraft reaches FL370 due to weight reduction?

Thank you very much, I would appreciate a reply asap as I have an interview in a few days.

[de facto]

On the 737,it is about 6 kts if HW/1000 ft off the optimum.

[Alex Whittingham]

The optimum altitude is not going to increase by 2000ft in the time it takes to climb. To me the question sounds confused. The optimum altitude slowly increases as the aircraft weight reduces and you would normally step climb to above the optimum, say FL350 if the optimum was FL330, then keep that level as the optimum rises through FL350 to about FL370, then step up again to FL390 and repeat the process.

My guess is that the 'optimum altitude' in brackets after FL330 is misleading, and that they mean that you are cruising at FL330, the optimum is FL350 and you are capable of FL370.

Unfortunately I can't help you with the use of table itself. Which aircraft type?

[fullandfree]

Wind Altitude Trade Table

Hi there

OK, I am using the 737-800 trade table as an example and will do my best here to clarify this.

First thing; the trade tables (from which you have been given some figures, though of course I have no idea for what aircraft - it matters not) allow you to calculate whether you are able to maintain your present range capability at a new altitude. They do this by allowing you to calculate the so-called 'break-even' wind at the new altitude - the wind at which you will be no worse off (in terms of range capability) climbing or descending to that new altitude.

Step 1: Read the 'wind factors' for the present altitude and new altitudes from the table: these are what you have been given:

F370: 0
F330: 10
F290: 38
headwind 100kts (-100)

Step 2: Determine the 'difference' (new altitude wind factor Minus present altitude wind factor) - answer may be positive or negative:

Step 3: Break even wind at new altitude is:
present altitude wind plus 'difference'

So,difference between F330 and F370:
'New altitude wind factor - present altitude wind factor':
0 (F370) - 10 (F330) Difference is -10 (minus 10)
so, the 'break even' wind is -100 (current headwind) plus (-10) = -110 kts

In other words, if you were to climb to F370 and the headwind component was 110 kts, you would have the same range capability as at F330. If the headwind at F370 was less than 100kts, then you would have a better range capability than you had at F330.

Doing the same for F290:
'New altitude wind factor - present altitude wind factor':
38 (F290) - 10 (F330) = 28: this time the Difference is +28
so, the 'break even' wind is -100 plus (+28) = 72 kts
So, in this case, so long as the headwind component at F290 was no greater than 72kts, you would have the same or better range capability compared to the range capability at F330.

Just take care on the 'pluses' and 'minuses' and you should be fine.

So, in the example, you would not gain anything by climbing to 370 as you would only achieve the same range capability as at 330; however, by descending into the 50 kts headwind at 290 you would increase your range capability. So, the answer would appear to be to descend. This makes sense in the context of the interview - you would naturally expect your range capability to improve by climbing so would perhaps instinctively opt for that answer, although in the case it would be wrong.

Hope this helps - it is my understanding of trade tables anyhow.

Good luck for the interview.

Cheers

[fullandfree]

Whoops!! Just seen the date of the original post on here - guess we missed the interview then!!