pressure altitude etc.

[ranklein]

Hi,

I'm trying to get a fast & simple way of calculating these questions:

1. Indicated alt. 10000ft.OAT -20C.You set 29.62" for a station which is 2500ft high.What's your actual altitude?
2. At ISA -25C, how much altitude should be added to MEA?
3.What's SSB?
4.TAT is -30c.Adiabatic temp. rise is 10 degrees, what's SAT?

Thanks!!

[MrBernoulli]

Isn't the answer to 4:

TAT less the adiabatic rise = SAT
-30 + 10 = -20 ?

[Old Smokey]

.1. True Altitude = 9856.9 ft (7086.9 ft above station). This all assumes that the QNH was given by the station and correct for the station elevation.

.2. It depends upon the MEA. True altitude error is a multiple of the altitude difference between the reference station and the mean temperature of the column of air to the station Vs the ISA temperature of the column of air to the station. It is not a simple additive or subtractive.

.3. Single Side Band (As applied to HF radio).

.4. SAT = -40°C (Your TAS must be 275.4 Kt or M = 0.463 for a 10°C RAM Rise). Mr. Bernoulli, TAT is always higher than SAT.

Regards,

Old Smokey

[MrBernoulli]

Er, yes of course!!!!!!

Sorry Smokey, brain is completely mashed today. You are, as always, absolutely correct. Think I'll find another cup of coffee ....................

[yankiwi]

The Cathay interview book that these questions come from is not completely correct . There are a few errors in it here and there. Like the answer to your question #4. The book gives the answer as -20c.

Old Smokey, I have learned an enormous amount from you on these forums, and for that I thank you. But with reference to #1 is it not closer to 9556ft?

My math for that goes like this.
OAT is ISA -15, using 4% per 10deg, we use 6% of the indicated altitude minus the station elevation.
10000ft-2500ft=7500ft
6% of 7500ft is roughly 450ft.

10000-450=9550.........

Have I been doing this wrong?

If I could ask for your workings on how you get your answer it would ease my confusion!

[ranklein]

yankiwi,

According to the answers in the bok you're right on the money!
So I gather you correct by 1% for every 10 degrees deviation from ISA?

R.

Let me correct myself:

4% for every 10 degress? again, the answer does show 9550ft.

Old smokey:

True Altitude = 9856.9 ft (7086.9 ft above station). This all assumes that the QNH was given by the station and correct for the station elevation.

Can you explain the calculation?

Thanks

[WaldoPepper]

Sent you an email but thought i'd post here as well so others can use or abuse ...

1. I get about 9,500 ft for this one. I just did it on the "wiz wheel" circular slide ruler thingy. Not sure of the math involved but I did know the math 10 years ago when i first studied my airline subjects.

2. This is the rule from ICAO documents:
ISA -15 to ISA -30 deg = add 10% to the MEA
< ISA -30 deg = add 20% to the MEA

so your answer would be to add 10% to the MEA.

3. SSB is "Single Side Band". It's used in radar and more frequently in HF radios. Not knowing much about the physics of radio waves what I do know i'll try and explain:

Sidebands are created whenever a carrier wave is modulated by a frequency lower than itself. If the carrier frequency is Amplitude Modulated (AM) at 500 Hz with an audio tone of 2 Hz, 2 sidebands are created, one will be at 498 Hz, the other at 502 Hz, with a 4 Hz split between the 2 sidebands. The carrier signal does not carry any info, it is the sidebands that carry the information. Because there are 2 of them, and are mirror images of each other, only one side band needs to be sent. By sending this Single Side Band (SSB) either more power can be used to send it and bandwidth saved, or power can be saved.

Summing up, Transmission is better because more power can be used to tranmsit the signal in a narrower bandwidth.

4. TAT=SAT + RAM rise. therefore the answer would be -40.

WP

[Old Smokey]

Sorry for the late reply to several queries. Don’t forget that the True Altitude correction applies only to the 7,500 ft column of air between the Reference Station and the aircraft.

.1. ISA Temperature at 10,000 ft = 15 – 10 X 1.9812 = -4.812°C
.2. ISA Deviation at 10,000 ft = OAT – ISA = –20 – - 4.812 = ISA – 15.188
.3. ISA Temperature at 2,5000 ft = 15 – 5 X 1.9812 = +10.047°C
.4. ISA Temperature of Mean column of air above Reference Station = (-4.812+10.047) / 2 = + 2.6175°C + 273.15 = 275.7675°A(K)
.5. Actual Temperature of Mean column of air above Reference Station = 275.7675°A – 15.188 = 260.5795°A(K)
.6. True Altitude above Reference Station = 7,500 ft X 260.5795 / 275.7675 = 7086.9 ft
.7. True Altitude Above Sea Level = 2500 + 7086.9 = 9586.9 ft

If you consider that the QNH of 29.62” EQUALLY affects the Station Pressure Height and the Aircraft Pressure Height (to 2,780 and 10,280 ft respectively) it makes a VERY slight difference to the calculation. If this is considered, the True Altitude above the station becomes 7086.1 ft (9586.1 ft AMSL), an error of a mere 0.8 feet.

Regards,

Old Smokey