How to use Weight Altitude Temperature (WAT) limit chart

[ktrwcm2014]

Can someone teach me how to use this WAT chart? I'm going through the Aerocourse ATPL workbook and I'm getting totally different numbers when I do the calculations even though they seem like simple calculations. Answers are bolded

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30. Given the following info, determine the max allowable t/o weight and EPR for a heavy jet transport a/c departing Calgary (refer to 5-12)
Active rwy 16 wind 160/10kts QNH 30.12” Temp 20’C Packs On
a) 158.8kgs, 1.52 EPR
b) 159.7kgs, 1.52 EPR
c) 172.1kgs, 1.52 EPR
d) 173.1kgs, 1.52 EPR
I got 174.1kgs for Flap 5 which is more than the answer 172.1kgs and 174.1 isn't even one of the choices. It says on the bottom right corner that runway limit weight may be greater than climb limit weight. With that being said, I went with the next greatest answer D but it's wrong.

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31. Determine the max t/o weight and EPR for a B767 if the alt setting is 29.82” and the temp is 32’C w/ a HW component of 7kts favoring rwy 10 (refer 5-12)
a) 149.6kgs, 1.42 EPR, flaps 5
b) 161.1kgs, 1.42 EPR, flaps 5
c) 150.4kgs, 1.42 EPR, flaps 20
d) 150.9kgs, 1.42 EPR, flaps 20

calc for Q31.jpg (144.31 KiB) Viewed 2980 times

32. Determine the max allowable t/o weight for a heavy jet transport a/c under the following cond. (refer 5-12)
Active rwy 16, winds 340/04kts, QNH 29.43”, Temp 0’C, light snow
a) 170.6kgs, 1.60 EPR
b)171.4kgs, 1.60 EPR
c) 172.2kgs, 1.60 EPR
d) 172.9kgs, 1.60 EPR

I'm not going to bother showing my work for Q32 b/c I get an answer that isn't one of the choices.

It'd be greatly appreciated if anyone can help me understand what this chart means and how to use this chart.

Cheers

[photofly]

I’ll guess that in the first case, there’s a 5 flap climb weight limit of 172.1 which overrides the runway weight limit. You can’t take off heavier than your climb limit, regardless of the runway limit.

[wordstwice]

Like the other person mentioned, the first question you are limited by your climb performance.

For the other two, did you notice how the first question mentioned Pack On for takeoff but the other two questions don’t say Pack on.

Try redoing them with the pack off credit. I did all three questions and came to the correct answers.

[NotDirty!]

It looks like they don’t expect you to interpolate the headwind credits. So for 7 kts of headwind, use 5 kts, etc. You will probably find that in the “real world” most pilots will be conservative with HW credit, because between the time you get your WAT data and when you actually take off, the wind may die down or shift so you no longer have 7 kts on the nose. But that makes these exam questions harder, because you have to guess how conservative they expect you to be.

As a rule of thumb, if you want to maximize takeoff performance, you want as much of the air entering the engine as possible to be used as thrust. Thus for “max performance” figures, you always use packs off (or on APU bleed, which allows you to continue to have air conditioning, but without bleeding any air off the engines).

The last question, with light snow mentioned, is prompting you to use numbers with engine anti ice on. Because the engine anti ice uses engine bleed air, there is a thrust penalty. And because you will be operating in visible moisture, below 10C TAT, you need that engine anti ice. Wing anti ice, however, wouldn’t be used in those conditions, so you only use the engine anti ice penalty.

[ktrwcm2014]

That makes sense. Thank you so much everyone

[Korbinian]

Hi guys, I know this is an old thread, but I am stuck on question 32. as well, and I cant seem to get the correct answer of 170.6, the closest I can get is 170.5. Would someone who has gotten this question correct be able to show how they arrived at that number? Thanks in advanced.