TRUE ALTITUDE = ALTITUDE ON QNH (ISA DEVIATION x 4 x PRESSURE ALTITUDE ÷ 1000)

Alt on QNH = 12000ft

Isa deviation = 18 ( supposed to be -9 but it is 9)

Ta =[12000plus ( 18x4x12000)/1000 ]

Ta = 12864

Ac is flying into high pressure

When flying from

Low to high altimeter will under-read

but True altitude is same

The true altitude of this surface will depend on the density of the air below flying from marseille  where the qnh is lower to Palma where the qnh is higher you would normally expect your true altitude to increase since true altitude remains constant it must be colder in Palma

molecules in cold air are closely packed

molecules in warm air are loosely packed

a column of cold air on the left, and a column of warm air on the right. Notice that, since cold air causes pressure to fall more rapidly with height, the pressure levels in the cold air column are compressed towards the Earth’s surface, whereas, in the warm air column, they expand away from the surface

.

If temperature is warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

If temperature is warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

molecules in cold air are closely packed

molecules in warm air are loosely packed

a column of cold air on the left, and a column of warm air on the right. Notice that, since cold air causes pressure to fall more rapidly with height, the pressure levels in the cold air column are compressed towards the Earth’s surface, whereas, in the warm air column, they expand away from the surface

.

if 1013 is set on a cold day the aircraft will over read elevation than actual

Remember 

HIGH LOW HIGH (over read) 

WARM COLD OVEREAD

If temperature is warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

molecules in cold air are closely packed

molecules in warm air are loosely packed

a column of cold air on the left, and a column of warm air on the right. Notice that, since cold air causes pressure to fall more rapidly with height, the pressure levels in the cold air column are compressed towards the Earth’s surface, whereas, in the warm air column, they expand away from the surface

.

if 1013 is set on a cold day the aircraft will over read elevation than actual

Remember 

HIGH LOW HIGH (over read) 

WARM COLD OVEREAD

As we know pressure falls with height, 1013 hpa will be found above 1030 in the atmosphere , if 1013 is at sea level , 1030 is below sea level,  with a difference of 17 hpa , every feet is 27 hpa , so 17 hpa is 459 feet,as you can see from the diagram 1013 hpa set on altimeter will give less elevation, it drops by 459 feet

lowest forecast QNH  provided by the Met. Office, is used to ensure safe terrain clearance. It is the value below which QNH is forecast not to fall in a given period and area.

The value should be lower than the actual QNH anywhere in the area, and if set on the sub-scale, regional QNH will cause the altimeter to under-read

(the altimeter showing aircraft to be lower than it actually is).

To know the temperature of the air you need to know two things, the indicated altitude on QNH and the true altitude. You know this because if it’s warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

In the question then, whilst you have the true altitude they have not given you the indicated on QNH. They have given you a flight level which you can use to work out the indicated on QNH. So, get off the flight level 1013 and onto 1003.

You now see that the INDICATED on QNH is 9730 FT. Notice that this is less than the TRUE altitude of 10,000ft. Therefore the TRUE altitude is more than the INDICATED altitude.

Looking back to the second paragraph this can only happen if the air is warmer!

Airfield QNH is obtained by correcting a measured QFE to sea level using ISA regardless of the temperature structure of the atmosphere. As your altimeter is calibrated using ISA, it will indicate altitude correctly at the airfield reference point

A= closely packed iso bars indicate low pressure

B= Widely spaced isobars indicate high pressure

flying from low to high pressure true altitude will increase

8 metre above msl pressure reduces by 1 hpa

200 metre above msl pressure increases by 25 hpa

Current pressure 1000 plus 25 gives qnh

Qnh is 1013 only under isa conditions

which is hypothetical

If temperature is warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

Under normal conditions, pressure varies by 1 hPa per 27 feet.

1038 hPa is (1038-1013.25) = 24.75 hPa higher than standard

24.75 x 27 = 668.25 ft

So FL75 will be at 7500 plus 668.25 = 8168.25 ft msl
So, 8168.25 – 1500 = 6668.25 ft agl, rounded up to 6670 ft agl

Forget about the part in which aircraft is at aerodrome with 1023 setting

at pressure altitude

altitude Measured from 1013 (SPS)setting

will be less than altitude measured at 1023 setting

True altitude in High pressure or warm atmosphere is always more than cold/low ,

hence qnh more than 1013 and temp more than isa

“watch out below when you are low”

Since both the places have same pressure it is obvious that we are flying to a warmer place

true altitude increases in warmer atmosphere

If the QNH was 1013mb, you’d be at 13,500 feet when at FL135. if you increased the subscale setting to 1019 the indication would increase, but as per radio altimeter it remains 13500 , this means that air mass was cold as true altitude decreases in cold air

As a thumb rule

if it’s warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

altimeter shows indicated altitude

hence lower altitude than summit

1013 will be below 993 in atmosphere

20 hpa difference will add 540 feet

Under normal conditions, pressure varies by 1 hPa per 27 feet.

hence add 540 to current altitude

1080 feet with 1013 set

True Altitude = Pressure  Altitude plus(ISA Deviation × 4/1000 × Indicated Altitude)

if deviation is 0 true altitude is equal to pressure altitude

which is only in international standard condition

In simple words

qfe is pressure at the airfield

qnh is pressure at the sea level right below the airport

setting QFE will give the height of the aircraft from the airfield

setting QNH will give  height of the aircraft from the sea level( elevation)

if the airfield is below the sea level atmosphere will exert more pressure on the air field than sea level

Here we must apply two corrections. A barometric difference and a temperature difference. The baro comes first.

FL160 is a pressure altitude of 16 000 feet with the Kohlsman window (subscale) in the altimeter set to 1013.25 hPa (29.92″) in a standard atmosphere. If the pressure datum (QNH) for mean sea level is lower than ISA then our true altitude is LESS than indicated. (High to low, look out below.)

Here since the QNH is less than standard, by about 10 hPa, we are 270 feet lower than the altimeter says. If we were to remain in level flight and put 1003 into the Kohlsman window, the altitude indicated would be 15 730 feet. Presuming no other ISA deviations, of course.

However we do have a temperature deviation. In ISA there would be a 31.5°C drop from MSL to 15 730 feet (approximately 2°C lapse rate per thousand feet ), so 15°C minus 31.5°C is  -16.5°C. The OAT being -27°C means it is 10.5°C colder than ISA.

True Altitude = Indicated Altitude plus(ISA Deviation × 4/1000 × Indicated Altitude)

The temperature correction was (15 730 x0.04 x -10.5/10 )= -661 feet which puts us at about 15 069 feet (although the altimeter tells us 16 thousand).

As area C is closely packed with isobars it is a low pressure area and D is a higher pressure area than C

when you fly from high to low altimeter overreads

also in a low Pressure area indicated altitude is more than true altitude hence true altitude is lower as indicated altitude is same.

FL120 minus FL060 is 6000 feet in ISA. The question relies upon you knowing the altimeter correction for temperature deviations from ISA. The accepted appoximation is 4% height difference for every 10°C below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above -15°C.

6000 feet x0.04 x 10/10 = 240 feet

.The 0.04 is the 4%, the 10 is the ABOVE ISA deviation.

The altimeter will still register the difference as 6000 feet but in reality we have an approximate depth of 6240 feet

Warmer than ISA — altimeter under-reads.

Colder than ISA — altimeter over-reads.

FL120 minus FL060 is 6000 feet in ISA. The question relies upon you knowing the altimeter correction for temperature deviations from ISA.

The accepted appoximation is 4% height difference for every 10°C below standard temperature as measured at the altimeter setting source.

This is safe for all altimeter setting source altitudes for temperatures above -15°C.

6000 feet x0.04 x -10/10 = 240 feet

.The 0.04 is the 4%, the -10 is the below ISA deviation.

The altimeter will still register the difference as 6000 feet but in reality we have an approximate depth of  5760 feet

Warmer than ISA — altimeter under-reads.

Colder than ISA — altimeter over-reads.

if it’s warmer than ISA

then TRUE > INDICATED

and if it’s colder than ISA

then TRUE < INDICATED.

True Altitude = Indicated Altitude plus (ISA Deviation × 4/1000 × Indicated Altitude)

with all the info given we can find out the isa deviation of temperature but not the temperature

Here we must apply two corrections. A barometric difference and a temperature difference. The baro comes first.

FL100 is a pressure altitude of 10 000 feet with the Kohlsman window (subscale) in the altimeter set to 1013.25 hPa (29.92″) in a standard atmosphere. If the pressure datum (QNH) for mean sea level is lower than ISA then our true altitude is LESS than indicated. (High to low, look out below.)

Here since the QNH is less than standard, by about 30 hPa, we are 810 feet lower than the altimeter says. If we were to remain in level flight and put into the Kohlsman window, the altitude indicated would be 9190 feet. Presuming no other ISA deviations, of course.

However we do have a temperature deviation. It is 15°C colder than ISA.

The temperature correction was

True Altitude = Indicated Altitude plus(ISA Deviation × 4/1000 × Indicated Altitude)

(4x -15×10 )= -600 feet

Add it to 9190

which puts us at about 8590 feet (although the altimeter tells us 10 thousand).

Qnh and QFE can be determined from each other if elevation is known ,

using

Under normal conditions, pressure varies by 1 hPa per 27 feet.

if airfield is at sea level, I.e “0”elevation

then QFE = QNH

In N hemisphere

Starboard drift= flying into low pressure

Port drift= flying into high pressure

In S hemisphere

Starboard drift= flying into high pressure

Port drift= flying into  low pressure

flying from east to west

winds from north

port drift

in N hemisphere

port drift means flying Into high pressure

high pressure gives you gain in altitude

1240 feet with the Kohlsman window (subscale) in the altimeter set to 1008 hPa in a standard atmosphere. If the pressure datum is 1013Here since the QNH is more , by about 5 hPa, we are 135  feet higher than the altimeter says which is 1375

True Altitude = indicated  Altitude plus(ISA Deviation × 4/1000 × Indicated Altitude)

I.a = 20000

isa deviation = -15

ISA Deviation × 4/1000 × Indicated Altitude=  -15×0.004×20000 =-1200

True Altitude = indicated  Altitude plus(ISA Deviation × 4/1000 × Indicated Altitude)

=20000-1200= 18800

2nd

apply barometric correction

FL200 is a pressure altitude of 20000 feet with the Kohlsman window (subscale) in the altimeter set to 1013.25 hPa (29.92″) in a standard atmosphere. If the pressure datum (QNH) for mean sea level is higher than ISA then our true altitude is High than indicated.

Here since the QNH is more  than standard, by about 20 hPa, we are 540 feet higher than the altimeter says

Add 18800 plus 540