Question for CPL pilots ;)

[sidh 29]

Heys guys,

I have come up with few questions 

 

Q.1 Temperature at 2KM is 5*C what is ISA deviation ? Hint(Actual - ISA) Given that temperature falls 6.5*C/1KM while going up.

 

Q.2 In actual atmosphere temp at 19KM is -60*C. Find ISA Deviation?Given that temperature falls 6.5*C/1KM while going up.

 

Also give reasons/proof for your answerers.. 

BTW these are real type Exam questions for CPL

[Murmur 3,941]

I don't have a CPL, so waiting for a real CPL to answer, I'll give mine:

 

Q.1 ISA SL temperature is 15C. In this case, temperature at SL will be: 5 + (6.5 x 2) = 18C, so that makes: 18-15 = +3C ISA deviation.

 

Q.2 I don't know if this is the correct answer, maybe not. However, ISA atmosphere has constant temperature above 11km. So, in this case you'll have -60C at 11km, and -60 + (6.5 x 11) = 11.5C at SL, so that makes: 11.5-15 = -3.5C ISA deviation.

[bjratchf 80]

I thought it fell around 2º per 1000 feet... Unless the exam is using some funny values (like km!)

[mgh 89]

Feet are the funny values in this case, By definition in the temperature fall by 6.5^OC/km (0.0065^OC/m) below 11km

[sidh 29]

I thought it fell around 2º per 1000 feet... Unless the exam is using some funny values (like km!)

2*C/1000 feet = 3.5*F/1000 feet = 6.5*C/KM

[sidh 29]

I don't have a CPL, so waiting for a real CPL to answer, I'll give mine:

 

Q.1 ISA SL temperature is 15C. In this case, temperature at SL will be: 5 + (6.5 x 2) = 18C, so that makes: 18-15 = +3C ISA deviation.

 

Q.2 I don't know if this is the correct answer, maybe not. However, ISA atmosphere has constant temperature above 11km. So, in this case you'll have -60C at 11km, and -60 + (6.5 x 11) = 11.5C at SL, so that makes: 11.5-15 = -3.5C ISA deviation.

I think that is right too but check my method

Ans.1) The formula for Finding ISA Deviation is = Actual Temperature - ISA Temp.

We know the Actual temperature that is 5*C but we don't know the ISA temp.

 

We also know that

ISA at Sea level is +15*C with increase in altitude temp reduces at rate of 6.5*C/1km 

 

So ISA Temp for 2 km will be 15-6.5*2=15-13 ... = 2*C

 

So now I know the ISA temprature I will simply use ISA dev formula = Actual Temp(Given 5*C) - ISA temp(2*C)= 3*C 

[Murmur 3,941]

Yes, the two methods are absolutely equivalent.

[mgh 89]

2*C/1000 feet = 3.5*F/1000 feet = 6.5*C/KM

 

^OF are irrelevant here and, anyway, 2^OC = 3.6^OF - not 3.5^OF

There are 3.28083 feet / metre so the relationship is simply

6.5^OC / 1000m = (6.5 / 3.28083) ^OC / 1000ft = 1.9812^OC / 1000ft.

[bjratchf 80]

Ah ok. I just look at the FMC these days...

[sidh 29]

^OF are irrelevant here and, anyway, 2^OC = 3.6^OF - not 3.5^OF

There are 3.28083 feet / metre so the relationship is simply

6.5^OC / 1000m = (6.5 / 3.28083) ^OC / 1000ft = 1.9812^OC / 1000ft.

We take the approx value in all these case so to make calculations easier. On some books you will read value a change by few points so it's considered the same.

 

Example on some books you will see percentage of Carbon dioxide to be 0.03% and on some 0.05%. It varies but since it is in a very small amount it is considered the same

[mgh 89]

I agree the approximations are acceptable for day-to-day use but 2^OC does not equal 3.5^OF as you wrote.

[Cessnaflyer 56]

After getting my ATP I've used 0 of the equations required. Through theory of flight I used to be able to calculate nearly everything by memory but once you start flying the only thing we care about is if we are starting our descent so it is a nice stable 1500 fpm descent. 

[sidh 29]

After getting my ATP I've used 0 of the equations required. Through theory of flight I used to be able to calculate nearly everything by memory but once you start flying the only thing we care about is if we are starting our descent so it is a nice stable 1500 fpm descent. 

That is right but DGCA and JAA are different from FAA .However they are co-related (ofcourse) but much more complex and detailed

[Cessnaflyer 56]

That is right but DGCA and JAA are different from FAA .However they are co-related (ofcourse) but much more complex and detailed

I know the FAA tests are a joke, but Theory of Flight classes are no joke. We would take a whole 4 hour class session working in minutia just to find how a rotation or stall speed would be changed.

[bjratchf 80]

I'm always impressed by how much knowledge the JAA tests required. It basically is a degree's worth of stuff! The tests are hard, no joke, and require diligent study.

 

That being said, you'll use 1% of it actually flying!