Some Wx&tech. questions unsolved...

[Jetperf]

Preparing for the EK interview, I need help to sovle some Wx/tech questions... a Big Thanks!!!

1/ Density alt calculation: temp. correction.
sometimes we use 118ft/1C, sometimes 4%/10C of the alt to be corrected...???

2/ EPR during T/O roll increase or decrease?
(after carefull observation on several T/O on MD11 I saw no change...)

3/Mach Crit is the speed at which
a) the shockwave leaves the wing
b) the first swockwave forms on the wing

4/Ws / microburst is associated with
a) turbulence
b) vetical develop clouds
c) frontal occlusion

5/ outline the change in the Center of pressure as an acft increases speed past Mcrit

[Eckhard]

Hi Jetperf,

I'm not an expert in any of your questions but you haven't had a reply in 9 hours, so here goes......

1/ 4%/10C deviation of the airmass beneath the aircraft.
(or 4ft per degree C per 1000ft)

2/ Modern engines will hold constant EPR but I think on older ones the EPR would decrease. That's why you have to have the thrust set by a certain speed.

3/ b)

4/ b)

5/ the CofP moves aft (hence 'mach tuck' and the need for mach trimmers on some types).

Hope this helps and good luck with the interview!

[Borat The Great]

Perhaps I can help with number 1.

I think density altitude is calculated by multiplying the ISA deviation in Deg C. by 118 then that figure is added or subtracted to the aircrafts pressure altitude to get density altitude, which is only really a performance consideration.

The 4Deg/1000ft thing is TRUE altitude which is you actual altitude if you got out and measured it with a stick and not what the altimeter says( due to temp error). Basically i think it means that the atimeter will show an error of 4ft for every 1000ft of altitude AND degree deviation from ISA.

So at 10000ft if temp is -20 deg.

thats 4x10x15 (ISA @10000 is -5)

600ft. and its colder so you are 600ft lower than your altimeter reads!