Cold temperature errors.

[NeverEnding]

Good evening!

Was hoping that someone might be able to provide me with an explanation as to how to figure out the following problems. I'm having a little bit of difficulty, and I'm sure its something extremely obvious that I am overlooking.
Any guidance would be much appreciated!
Thanks in advance!

1) An aircraft departs an airport that has an elevation of 2000' ASL and levels off at 6000' ASL which is the MEA for a victor airway south of the airport. The revised altimeter setting is 30.12" and the OAT at 6000' is -32C. What would be the ground clearance of the aircraft when corssing the 4950ft ASL ridge south of the airport?
a) At least 1000 ft.
b) Approximately 750 ft.
c) Approximately 490 ft.
d) Less than 150 ft.

2) An aircraft departs from Cranbrook Airport (CYXC 3082' ASL) and levels off at 11000 ft ASL which is the MEA for V303 east of Cranbrook. The revised altimeter setting is 30.22" and the OAT at 11000 is -40C. What would be the ground clearance of this aircraft when crossing the 9220 ft ASL moutain ridge east of Cranbrook?
a) 1000 ft
b) 740 ft
c) 400 ft
d) 200 ft

3) While flying in the standard pressure region at FL50, your outside air temperature gauge reads -40C. With a current altimeter setting of 30.12 inches, what would be your actual height above an airport with a field elevation of 2000 feet ASL?
a) approx. 2250 ft
b) approx. 2625 ft
c) approx. 3200 ft
d) approx. 3575 ft

(All questions were taken from the AEROCOURSE ATPL Workbook)

[co-joe]

Temp difference from standard X 4 X your altitude in thousands = a rough number to apply to your altitude.

Or use the chart in the CAP GEN, and CFS

[SAR_YQQ]

Here's some help....

I will lead you through the steps towards gaining the answer for #1 and you can use it to figure out the rest.

1) An aircraft departs an airport that has an elevation of 2000' ASL and levels off at 6000' ASL which is the MEA for a victor airway south of the airport. The revised altimeter setting is 30.12" and the OAT at 6000' is -32C. What would be the ground clearance of the aircraft when corssing the 4950ft ASL ridge south of the airport?

Pertinent details:

Airport Elevation for Altimeter Setting = 2000' asl
Indicated Altitude = 6000' asl
OAT = -32C

The first thing that needs to be done is to determine what ISA you are currently experiencing at 6,000' ASL. Easy way to do this is to take the standard adiabatic lapse rate of 2C/1000' and apply it to an ISA situation.
(reminder, ISA at sea level is 15C, 29.92)
Therefore, at 6,000' ASL the temperature should be 12C cooler than it is at sea level (2C/1000'). So 15C(sea level) - 12C (ISA temperature drop) = 3C. According to your example, it is -32C at 6000'ASL. This is quite a bit colder than ISA! How many degrees colder is it? +3C to -32C is a 35C temperature spread. This is our first number - it is ISA -35.

What is the difference between the current aircraft altitude and the station altitude of the current altimeter setting? In this example it is 4,000'.

We now have all the numbers required for this example.

(ISA difference x 4) x column of air/ 1000

(-35 x 4) x 4000/1000

(-140) x 4

= -560' (this is how much your altimeter is under reading)

Therefore your true altitude is 6,000 - 560 = 5440' ASL

You will then cross the 4950' ASL ridge at:

5440' - 4950' = 490 '

(C) is the correct answer.

[NeverEnding]

Hey thanks a lot for the replies! Managed to get it figured out now...I also finally found some guidance in a tiny corner of the AIM.
I tried using the CAP GEN, but I wasn't coming up with the answers that seemed to make sense. Seems like the X4 rule works best.
Again, thanks for the help!