Atpl System Question

[GNR]

just having a little trouble...

for a constant IAS and constant FL, Mach number is affected by changes in air temperature.

true or false, and explain why

[GNR]

answer is false, and i found why

copy and pasted from a website:

When a constant Mach number is maintained at a constant level, penetrating an area of colder air will see NO change in IAS, but a decrease in TAS by approximately 1 kt per degree C. The reverse is the case when OAT is increasing, IAS unchanged, and TAS increasing.

This is because the IAS gauge uses the same pitot and static port values as does the Machmeter.
Both therefore will be effected equally by temperature/density changes.
TAS decreases in cold air because the speed of sound decreases in colder air. When we maintain a fixed Mach number, we maintain a certain percentage value of the speed of sound. Therefore, if the speed of sound decreases we will also get a decrease in TAS. Effectively you are flying at a fixed percentage of a smaller number. Refer to diagram below.

[ksa5223]

Constant IAS @ Constant FL equates to one given Mach Number. Any change in temperature from this point will only change your TAS

Mach number is independent of temperature because:

MACH NO = TAS/LSS ( Local Speed of Sound)

TAS and LSS are both temperature dependent.

Therefore if both TAS & LSS increase / decrease with a change in temperature by the same amount your left with the same mach number.

[shamone]

Hey, I'm guessing that the difference in density between water/steel etc and air is so significant that the small differences found in density/pressure of air is considered nil?:confused: Remember 10m of water is equivalent to 1 atmosphere of air!!

Speed of sound is ONLY affected by temperature...