Help needed with CRP-5 questions!!
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Join Date: Jul 2002
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Help needed with CRP-5 questions!!
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!
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!
Join Date: Mar 2001
Location: Florida, USA
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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.!!
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.!!
Jet Blast Rat
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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
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
