Hi,

I have a quick question regarding a leak in the exhaust system.

I understand that a leak in the induction system causes the Upper deck Pressure to 'reduce', which is sensed and the waste gate gets adjusted (closes more) to re- direct more exhaust gas to the turbine which then increases the UDP back up to it's preset value (which is why it's hard to get noticed).

If there's a leak in the exhaust system however, am I right by saying that 'less' exhaust gas gets sent to the turbine (due to the leak) which in turn reduces the UDP which in turn is sensed and the waste gate gets closed more than usual to compensate?

I'm just having a bit of trouble visualising the exhaust system leak.

help would be appreciated.

Cheers

Hi Ray,
You are correct.

If there is a leak in the exhaust system upstream of the turbine then, for a given waste gate setting, less gas will be getting to the turbine because of the leak. The turbine doesn't spin as fast which means the impeller doesn't spin as fast and the upper deck pressure won't be as high as it should be. Therefore the waste gate will need to close more in order to get the right exhaust gas pressure going through the turbine to keep the upper deck MAP at the required level.

Exhaust leaks downstream of the turbine / waste gate have no effect on the upper deck pressure but are still a fire risk of course!

Cheers,

Rich

Hi Rich,

Thanks for clarifying on that.

Just 2 more quick questions.

would a leak upstream of the intake system (before the impeller) have no effect? Obviously a leak downstream of the impeller in the Upper Deck would have the effect as described above. Would we be examined on "upstream of the impeller" if it exists?

Lastly, I cant seem to grasp the difference between Full Throttle Height and Critical altitude as mentioned in the book, had a look at the forums as well and still no luck. To me they both seem the same: the height at which full throttle produces the rated boost and any climb above results in a loss of power - at this height I'm also assuming the waste gate would be fully closed (which is why no more power is available above it)

If you can clarify, that would be greatly appreciated.

Cheers again!

Ray

Hi Ray,

A leak upstream of the impeller would have no effect as far as I can see and as such it wouldn't make much sense to put it in the exam.

Full throttle height is simply the height at which you would need full throttle to achieve some specified manifold pressure and every manifold pressure has its own full throttle height. For example, the full throttle height for 20 "Hg might be 14000ft. That means at 14000ft, you will need full throttle to achieve a manifold pressure of 20"Hg. Any higher than that and even though you have full throttle, you will not be able to get 20"Hg in the inlet manifold.
In the same aircraft, the full throttle height for 17"Hg is going to be somewhat higher, probably about 2000ft higher for example. That would mean the full throttle height for 17"HG is 16000 ft.

The "Critical Altitude" is simply the full throttle height for rated boost. So, if the rated boost of the engine is 30"Hg, the height where full throttle is just giving you 30"Hg MAP is your critical altitude, or, in other words the full throttle height for the rated boost of 30"Hg.

An aircraft only has one Critical Altitude but it has many "full throttle heights" - one for every manifold pressure value.

Cheers,

Rich

Hi Rich,

A bit delayed but thanks for that. I ended up getting 85%.

To those studying - It's a bit different to the other CASA exams in terms of the wording of the questions. Bob's book covers everything you need to pass, if you have a thorough understanding of the topics you'll pass with no problems at all.
Make sure you read each question carefully and try and understand what it's asking you. When doing the practice questions in the book I find it helpful to read the question and think about how the various systems etc function and then go on to selecting the answer.

Good Luck.

Regards,
Ray