Above the tropopause temperature is constant. Speed of sound is depends from temperature. If temperature is constant, speed of sound is constant, so mach no. also remain the same. :)

Check this picture: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter1/graphics/vert_temp.gif
As I told, from tropopause till about 20km temperature is constant.

oh, i got tricked in this one , i was expecting a/c to descend further down to troposphere level :) thanks

Uhu, website contains some tricky questions, users so many times reports me they as wrong...
Most often are questions about binary coded decimals, users confuse bcd with simple binary. And human factors module contains question about how alcohol acts on reaction times.

thanks for the heads up, i'll focus more from now on-wards :D

Sir Dimky.. would u plz!! ans.these below Que...need ur help!THx! in advance..

A.In transonic speed range:
1.the centre of pressure movement may become ocillatory.
2.the position of the wing centre of pressure remains constant.
3.the centre of pressure initially moves forward then back.

B.The vertical fin of a single engined aircraft is:
1.parallel with both the longitudinal & vertical axis.
2.parallel with the longitudinal axis but not the vertical axis
3.parallel with vertical axis but not the longitudinal axis.

C.Wing loading is:
1. the maximum all up weight multiplied by the total wing area.
2.the maximum all up weight divided by the total wing area.
3.the ratio of the all up weight of the aircraft to its basic weight.

D.An engine which produces an efflux of high speed will be:
1.less efficient
2.speed of efflux has no affect on the engine efficiency.
3.more efficient

A=2,B=1,C=2,D=1.

Please be more specific with this question. Say "An aircraft flying above the tropopause descends at a constant True Airspeed. DURING THE DESCENT INTO THE THROPOPAUSE its Mach No. will"
The capital is to higlight the changed part. Change is needed so it is clear the plane is still into the thropopause when it's considered.