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Reverse thrust question

Old 8th Jan 2020, 13:40
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Join Date: Feb 2010
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Reverse thrust question

Hi

Just going through ATPL exam revision for Powerplant using latest cae oxford books. One of the answers doesn't seem right for reverse thrust (blocker doors).

Question: Once the blocker doors are fully deployed, with an increase in rpm, which of the following statements would be incorrect?

a. Forward thrust from the hot gases would increase.
b. Forward thrust from the hot gases would decrease.
c. Reverse thrust from the blocked air would increase.
d. TGT will increase.

I thought a. would be correct, as with blocker doors, the cold airstream only is reversed and not the hot air. So if you increase the RPM, then surely the forward thrust from the hot gases would increase in-line with an increase in turbine speed, although the overall net thrust would be in reverse due to the larger amount of cold air thrust from the fan? Or am I reading this incorrectly? The suggested correct answer is b.

Thanks
vw_nutter is offline  
Old 8th Jan 2020, 18:04
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Your understanding of the subject appears correct, but be careful with the wording. The quesion asks "which of the following statements would be incorrect?" So which one is wrong?

With thrust reverse fully deployed, the cold stream air (low bypass) thrust would increase, and the hot air (core) would still increase as that's where the thrust is produced.
So B) Forward thrust from the hot gases would decrease....is wrong, which is why it's right
TryingToAvoidCBs is offline  
Old 8th Jan 2020, 21:44
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Doh! Makes sense now. Must read the question! Thanks!
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Old 9th Jan 2020, 09:29
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FYI EASA have said that they will not (would hope not to) use 'which of the following is incorrect?' questions because they are so easy to misread. The modern equivalent (if this were an exam question, and it isn't) would be 'Select which 3 of the following statements are correct...'
Alex Whittingham is online now  
Old 9th Jan 2020, 18:50
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Originally Posted by Alex Whittingham View Post
FYI EASA have said that they will not (would hope not to) use 'which of the following is incorrect?' questions because they are so easy to misread. The modern equivalent (if this were an exam question, and it isn't) would be 'Select which 3 of the following statements are correct...'
Are they still using the same format of GNAV question asking "What is the true track of A from B". It used to catch everyone
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