Help needed with CRP-5 questions!!

[Kapooley]

Im stuck on these two questions:

An increase of 0.15 in Mach No results in an increase of 93 kt in TAS. If the temp dev from ISA is +9 c, the FL is??

a. FL200
b. FL220
c. FL130
d. FL150


I know the correct answer is b FL220, but im not sure how to work it.

Also I'm stuck with this one,

An aircraft is flying at 1100 kt TAS at FL650, a change of 0.1 Mach causes a change of 57 kt TAS. The temp dev from ISA at FL650 is?

a. -6 c
b. +6 c
c. -3 c
d. +3 c

The correct answer is -3

Again not sure how you work it out, any pointers greatly received!! Thanks!

[GoneWest]

K -

Local Speed of Sound = 38.954 x (square root of) Outside Air Temp in °K.

1) Find LSS - 93 (kts) divide by 0.15 (Mach Inc.) = 620 kts (LSS!!)

2) Find OAT °K using LSS formula.

620 = 38.954 x sq rt OAT °K

so; sq rt OAT °K = 620 divided by 38.954

so; sq rt OAT °K = 15.162

so; OAT °K = 15.162 squared = 253°K

3) 253°K = ISA +9, so ISA = 253 - 9, = 244°K

Remember, °C = 244 - 273 = -29°C (OAT ISA)

Level that corresponds to 244°K = FL220

On a CRP5...

Line 15 on the inner scale with 93 on the outer scale (above the "10" on the inner, read the LSS of 620 [just to check]).

Go to Airspeed window, above Mach No. Index Arrow read temp of "-20" (ISA + 9°C in your example), so

ISA = -29°C.

Altitude Window: Align Zero Feet with +15°C (ISA) - then read that -29°C equates to 22,000.

Flight Level 220.!!

[Send Clowns]

For the second one remember that mach is a ratio, therefore a linear scale. Hence a change of M 0.1 (i.e. 1/10th local speed of sound(LSS)) corresponding to a change of 57 kt TAS indicates that the LSS is 10 x 57 = 570 kts. The 1100 kts is a red herring (useless information put in to confuse, for the non-English here).

My CRP-5 is at work, but that should tell you the local temperature (-59°C by the formula, but check on the CRP-5 since this is a CRP question). Since ISA at FL 650 is -56.5°C then -3° deviation sounds about right.

Hope this helps, any more problems feel free to email/message me.

Richard Dale,
Gen Nav instructor, Bournemouth Commercial Flight Training Centre