Wind Altitude Trade

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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