View Full Version : 2 questions

9th Aug 2002, 21:10

A couple of questions for you chaps,

1) What is the biggest hazard in heavy rain on final approach?

2) outline the change in the centre of pressure as an aircraft increases speed past M crit?

Ta for now.:confused:

Genghis the Engineer
9th Aug 2002, 21:29
Couple more questions:-

(1) Why is it too difficult to use a meaningful title in your posts

(2) Why do you feel we want to answer what are clearly exam questions for you? Personally I'm quite capable of answering these, by getting my textbooks out, but can't entirely see why I should, when you could do it yourself?


9th Aug 2002, 22:30
Why thank you for your pointless post.

The reason I can't open my atpl books and refresh my memory is that I happen to work outside of the UK where my house and books are, so I thought I would use the tech forum on pprune, how silly of me to do that.:mad:

9th Aug 2002, 22:31
Oh, and they are not for an exam.

9th Aug 2002, 22:37
Well maybe you should know already then...?:rolleyes:

9th Aug 2002, 23:20
I had previously thought this boat was uninhabited, it is nice to know there are other crewmembers about. Recently it seems that the boards have been used as quick reference material for persons not wanting to do their homework, whether for interview preparations or an impending ATP test. It has become annoying to see multiple questions on the same subject, it would appear that even a simple electronic search is too bothersome. Before throwing stones in my own glass house, I shall quit my ranting.


1. In heavy rain the most dangerous condition on final approach is the potential of a microburst if talking about convective type clouds. In heavy rain induced by stratiform clouds the lift and drag penalty of the attached water film on laminar type wings, and the momentum penalty are dangerous aerodynamic hazards. Significant roughness occurs at rainfall rates exceeding 150 mm/hr, while the momentum penalty becomes apparent for large transport category aircraft at 500 mm/hr.

2. For conventional aircraft: As speed increases beyond Mcrit the center of pressure moves aft toward the np, the airfoil produces a nose down pitching moment that must be countered by a change in cg (pumping fuel) or stab setting.

9th Aug 2002, 23:21

Regardless of whether or not these are exam questions, I think that they are just as valid as any other question I have ever seen in pprune.

wrt the c of p movement above MCRIT.

Just above MCRIT, the first shockwaves will form, usually above the thickest part of the wings. These wiill cause an abrupt increase in pressure, which will reduce the lift generated by the area of wing behind the shockwave. This will cause the c of p to move sudddenly forward.

Continued acceleration will cause the shock waves to be pushed aft towards the trailing edge. At this stage it is important to note that the airflow over the area of wing in front of the shockwaves will be supersonic. The curvature of the wings behind the thickest point effectively forms a divergent duct, or expansion corner, through which this supersonic flow accelerates. So velocity continues to increase and pressure continues to decrease all the way to the shock waves. So as the shockwaves move towards the trailing edge, the lowest pressure and hence c of p moves back with them. The overall effect is that after its initial surge forward, the c of p moves aft to about the 50% chord point.

This is obviously a very simplified version of a complicated process, but I think it covers the essentials.

I'm not entirely sure about your other question, because it never rains in Dorset!!!

10th Aug 2002, 07:58
With regards to your first question, the biggest hazard on FINAL APPROACH would be a sharp fall in visibility.

Genghis the Engineer
10th Aug 2002, 08:20
Okay, okay, I was feeling a bit grumpy last night.

But, you did phrase the questions rather as if they'd just been copied straight off an exam paper or interview brief. And I defend my point that anybody posting on Pprune should make a bit of effort over their titles.

Perhaps if you'd posted with a title like "hazards on final approach", and said something like "Chaps and Chapesses, a bunch of us over here after a few nasty finals are trying to decide on the most serious hazards, and what we should do to head them off. My kick-off ideas are, but I'd appreciate everybody elses thoughts", you could set an example to those who really are just avoiding doing their own homework, as %mac said.


10th Aug 2002, 11:00
Thank you to all the people who helped me out with my questions.
I was always told if you don't know, ask.

10th Aug 2002, 12:06

I hope this experience hasn't put you off (but sadly I suspect it has).

Almost all human knowledge is already committed to print in one form or another. The trick of course is to find out where the bit you require is.

If pprune members were to avoid asking questions about all previously recorded information, they would be limited to matters of opinion.

The phrasing of the original question is of course a matter of choice. Any reader who finds he has been tricked into opening a "boring" string because it was "badly titled", can of course use the back button.

Alternatively, send your questions to [email protected] I may not have the answer, but will not quible about the validity of the question.

10th Aug 2002, 22:31
After having the opportunity to do countless heavy rain approchs at night into many of eastern canadas airports,I always found the most difficult part for me was how my depth and distance perceptcion would be affected for the landing,due to distortion from windsheild.Only one of many things you have to watch for ,turbulance,slipperyrunway,downdraughts,etc.But the really worst part was getting soaked while walking to pub after shift.

11th Aug 2002, 12:08
Thanks for your reply Keith. Nice to see some people like yourself still out there.