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# Altimetry and M&B confirmation

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# Altimetry and M&B confirmation

14th Oct 2023, 10:15

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Altimetry and M&B confirmation

Good morning / ... all,

in preparation for an interview I'm reviewing my atpl stuff amongst which are altimetry and M&B.
If possible, can someone confirm whether my thinking is correct:

1) Altimetry
Flying at FL 130
grid MORA 12.600 ft
QNH950 hPA
ISA conditions

Will we clear the obstacles?
I would say yes based on following calculation:

hPa correction: 1013 hPa - 950 hPa = 63 hPa = 1.890 ft;
FL130 = 13.000 ft
--> Pressure height = 13.000 ft + 1890 ft = 14.890 ft

My confusion: it mentions flying at FL 130 so do I then assume we have standard on the altimeter, clearing the MORA by 400 ft?

In addition, a small confirmation request:
let's say it's ISA + 4, which is warmer so higher, I'd have to add 480 ft (120 ft/ ° isa deviation), right?

2) M&B calculation
Given weights:
DOM
fuel (general fuel, so simple calculation (not taxi, trip,..)
MTOM
MLM
MZFW

my calculation

A] Get the lowest of MLM, MTOM and MZFW
B] Calculate Operating mass by adding DOM + FUEL
C] Subtract Operating mass from the lowest in A

Would this be correct?

Thanks

14th Oct 2023, 15:54

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M&B will never catch on ...
14th Oct 2023, 19:01

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1) No, you’re applying the correction in the wrong direction. QNH is below standard at 950hPa so at FL130 on QNE == 1013.2hPa you are not much over 11,000’ QNH, well below the terrain. Looking at it slightly differently, at sea level the altimeter would read 1,900’ / FL019 if you set 1013 on it, so it is over-reading by 1,900’ relative to the actual height of the terrain; with 13,000’ indicated you’d actually be at 11,100’.

2) There’s not quite enough information given - it’s considered normal (legal) to plan to land with some fuel on board and that is included when you’re checking against MLM. Need to know a bit more about the different elements of the “fuel” as this could affect the maximum traffic load.

M&B will never catch on ...
Yeah, it exists, but I’ve never heard of anyone actually using it...
15th Oct 2023, 09:50

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Originally Posted by FullWings
1) No, you’re applying the correction in the wrong direction. QNH is below standard at 950hPa so at FL130 on QNE == 1013.2hPa you are not much over 11,000’ QNH, well below the terrain. Looking at it slightly differently, at sea level the altimeter would read 1,900’ / FL019 if you set 1013 on it, so it is over-reading by 1,900’ relative to the actual height of the terrain; with 13,000’ indicated you’d actually be at 11,100’.

2) There’s not quite enough information given - it’s considered normal (legal) to plan to land with some fuel on board and that is included when you’re checking against MLM. Need to know a bit more about the different elements of the “fuel” as this could affect the maximum traffic load.

Yeah, it exists, but I’ve never heard of anyone actually using it...
1) Not really sure if I've got it down then. I'm not allowed to post links yet, but on pilotpracticeexams(dot)com(slash)altitude-calculator
example 3 where: alt. 2400 ft ; QNH 1011 (not looking to calculate density height, just want to focus on PH)
resulting PH is 2460 ft which would align with my calculation?
Could it be that I'm tripping over the fact that in my example it says flying at FL130 instead of altitude 13.000ft on QNH?

2) yeah I guess a bit more info is required but that's all I have for now. But I've got a reference with all the weights so i'll look into it further, thanks anyway :-)
M&B will never catch on ...
Yeah, it exists, but I’ve never heard of anyone actually using it...
not sure I'm following on this. Is it because off "mass and balance" instead of "weight and balance"?

Thanks
19th Oct 2023, 08:19

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Think about how a conventional pressure altimeter works: It is set up to tell you how high you are above a particular datum, which you set on the sub scale. It also assumes a Standard Atmosphere.

If you are at sea level, the QNH is 1013 and you have that set, the altimeter will read zero. If the QNH is 1011 and you have 1013 set, it will read 60’ because it is experiencing what the pressure would be at 60’ AMSL when the QNH is 1013. The altimeter is simply converting static pressure into altitude assuming the atmosphere is standard and the sub scale corrects for the actual pressure datum, QNH in this case. Which is why mis-setting the datum can be very dangerous.

A flight level is simply what the altimeter reads with 1013.2 set. When the sea level pressure is below 1013, flight levels are lower than what the altimeter would read on QNH and when the sea level pressure is above 1013, flight levels are higher than than what the altimeter would read on QNH.

Another way of looking at it is that if you fly around at a constant indication on your altimeter without changing the sub-scale setting, you are following a surface defined by equal pressure, the two dimensional equivalent of an isobar. As the QNH changes, you will move closer or further away from sea level. If you fly from high to low pressure, the surface you are following may go below sea (or ground level), which is not a normal habitat for aircraft...
19th Oct 2023, 09:17

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Originally Posted by QuebecLima
Good morning / ... all,

in preparation for an interview I'm reviewing my atpl stuff amongst which are altimetry and M&B.
If possible, can someone confirm whether my thinking is correct:

1) Altimetry
Flying at FL 130
grid MORA 12.600 ft
QNH950 hPA
ISA conditions

Will we clear the obstacles?
I would say yes based on following calculation:

hPa correction: 1013 hPa - 950 hPa = 63 hPa = 1.890 ft;
FL130 = 13.000 ft
--> Pressure height = 13.000 ft + 1890 ft = 14.890 ft

My confusion: it mentions flying at FL 130 so do I then assume we have standard on the altimeter, clearing the MORA by 400 ft?

In addition, a small confirmation request:
let's say it's ISA + 4, which is warmer so higher, I'd have to add 480 ft (120 ft/ ° isa deviation), right?

2) M&B calculation
Given weights:
DOM
fuel (general fuel, so simple calculation (not taxi, trip,..)
MTOM
MLM
MZFW

my calculation

A] Get the lowest of MLM, MTOM and MZFW
B] Calculate Operating mass by adding DOM + FUEL
C] Subtract Operating mass from the lowest in A

Would this be correct?

Thanks
When flying on STD (1013) so in Flight Levels remember that:

- If local QNH is low (so lower than your 1013) -> look out below!
- if local QNH is high (higher than your 1013) -> look happy the sky!

The rest is math.
19th Oct 2023, 11:20

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If you look at an altimeter when you turn the setting knob, turning anti-clockwise to select a lower pressure setting, will lower the indicated altitude.
I always found this to be an easy way of working this out.
19th Oct 2023, 14:32

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Let me take a shot at It. As someone already told you, you applied the correction in the wrong direction. You are flying FL's hence your altimeter is set to 1013,2 hPa. If you wind down that setting to your QNH of 950 hPa, guess what happens to your altimeter reading. Hint: you end up with 11.110 ft if I recall your math correctly. The MORA is 12.600ft and at that altitude, above 5.000 ft, it gives you 2000 ft clearance. Therefore the highest obstacle rises up to 10.600ft. At 11.110 ft you will clear that obstacle in a way that would grant you a tea no biscuits meeting at the Chief Pilot's office at any reputable Airline. But hey, you'll be alive unless you throw in some wind and low temperatures into the equation.

Now let's see If I can get the M&B quiz right.

You must meet the three following restrictions:

That gives you:

TOF = Takeoff fuel
trip = trip fuel
FOB = ramp fuel (off block total fuel on board)
MRM = Max Ramp Mass

You take the lowest figure and that will be your allowed max payload

Note: on lighter aircraft MRM=MTOM

Last edited by ant1; 19th Oct 2023 at 15:40. Reason: Addition
20th Oct 2023, 01:55
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An aside -

The majority of folks aver to the use of 30 ft/mb and 120 ft/C deg deviation when playing with pressure and density heights.

That's fine, providing that you keep in mind that these constants, in general, are wrong. For lower levels, they are adequate and fit for purpose. However, if you are playing with clearance considerations from the hard bits, the variation in pressure height calculation errors really need to be kept in mind.

A link to an explanation Bob Tait's Aviation Theory School - Altimetry Rates - Bob Tait's Aviation Theory School Forums
20th Oct 2023, 20:44

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Another important point is that things like airfield and terrain elevation in aviation are measured in feet (in most places) and these feet are always the same absolute length. A “foot” on your altimeter is only on very rare occasions equal to a foot as you would measure it with a ruler, due to the fact that pressure and temperature have a role to play.

Great for separating aircraft on the same pressure setting from each other, but not so great at separating aircraft from terrain which is why there are safety margins built in, plus corrections for atmospheric conditions significantly below ISA.
21st Oct 2023, 13:35

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Originally Posted by FullWings
Think about how a conventional pressure altimeter works: It is set up to tell you how high you are above a particular datum, which you set on the sub scale. It also assumes a Standard Atmosphere.

If you are at sea level, the QNH is 1013 and you have that set, the altimeter will read zero. If the QNH is 1011 and you have 1013 set, it will read 60’ because it is experiencing what the pressure would be at 60’ AMSL when the QNH is 1013. The altimeter is simply converting static pressure into altitude assuming the atmosphere is standard and the sub scale corrects for the actual pressure datum, QNH in this case. Which is why mis-setting the datum can be very dangerous.

A flight level is simply what the altimeter reads with 1013.2 set. When the sea level pressure is below 1013, flight levels are lower than what the altimeter would read on QNH and when the sea level pressure is above 1013, flight levels are higher than than what the altimeter would read on QNH.

Another way of looking at it is that if you fly around at a constant indication on your altimeter without changing the sub-scale setting, you are following a surface defined by equal pressure, the two dimensional equivalent of an isobar. As the QNH changes, you will move closer or further away from sea level. If you fly from high to low pressure, the surface you are following may go below sea (or ground level), which is not a normal habitat for aircraft...
Originally Posted by sonicbum
When flying on STD (1013) so in Flight Levels remember that:

- If local QNH is low (so lower than your 1013) -> look out below!
- if local QNH is high (higher than your 1013) -> look happy the sky!

The rest is math.
Originally Posted by dixi188
If you look at an altimeter when you turn the setting knob, turning anti-clockwise to select a lower pressure setting, will lower the indicated altitude.
I always found this to be an easy way of working this out.
Thanks guys, I think a made the 'click'. Really appreciate the effort you've put into it.

Just putting this out there if there's other people struggling with this: The combination of your explanations have helped me create a personal tool of solving these problems with a different example:

QNH 1020
3500 ft
OAT 10°C
Assuming 1 Hpa = 30ft
Assuming 1° ISA deviation = 120 ft
Assuming temperature drop = 2°C/1000ft

My Approach
To find out where the aircraft actually is I need:
- Pressure altitude
- Density altitude derived from pressure altitude

To find pressure altitude I need:
- QNH difference

What pressure am I starting from and where is it going to find what I need?
I need Pressure altitude so that would mean I need to 'go' to 1013.
starting at QNH 1020 and going to QNH 1013 means I'm deducting 7 Hpa which translates to 210 ft and since I'm deducting QNH I need to deduct 210 ft. ("High to low, look below")

What altitude am I starting from and where is it going to find what I need?
I still need Pressure altitude and I have 3500 ft on QNH 1020. We established we need to deduct as per above so : 2500 ft - 210 ft = 3290 ft
Pressure altitude = 3290 ft

To find density altitude I need:
- Pressure altitude
- ISA deviation.

ISA deviation can be derived from pressure altitude

ISA at Pressure altitude 3290 ft = 8,42°C
Expected temperature drop based on 2°C / 1000ft:
3290ft = 6,58°C temperature drop so temperature should be: 15°C - 6,58°C = 8,42°C in ISA conditions

ISA correction:
We have 10°C which is 1,58 °C warmer than ISA, which translates to 189,6 ft higher (120ft / °C ISA deviation) (warm = higher, cold = lower hence temperature corrections are required when in cold weather operations)
3290 ft (pressure altitude) + 189,6 ft derived from ISA deviation = 3479,6 ft
Density altitude = 3479,6 ft

Again, thanks for the help everyone !

Last edited by QuebecLima; 28th Oct 2023 at 11:40. Reason: minor formatting change
21st Oct 2023, 13:36

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Originally Posted by ant1
Let me take a shot at It. As someone already told you, you applied the correction in the wrong direction. You are flying FL's hence your altimeter is set to 1013,2 hPa. If you wind down that setting to your QNH of 950 hPa, guess what happens to your altimeter reading. Hint: you end up with 11.110 ft if I recall your math correctly. The MORA is 12.600ft and at that altitude, above 5.000 ft, it gives you 2000 ft clearance. Therefore the highest obstacle rises up to 10.600ft. At 11.110 ft you will clear that obstacle in a way that would grant you a tea no biscuits meeting at the Chief Pilot's office at any reputable Airline. But hey, you'll be alive unless you throw in some wind and low temperatures into the equation.

Now let's see If I can get the M&B quiz right.

You must meet the three following restrictions:

That gives you:

TOF = Takeoff fuel
trip = trip fuel
FOB = ramp fuel (off block total fuel on board)
MRM = Max Ramp Mass

You take the lowest figure and that will be your allowed max payload

Note: on lighter aircraft MRM=MTOM
Ah thanks, this is exactly what I needed !
21st Oct 2023, 13:38

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Originally Posted by john_tullamarine
An aside -

The majority of folks aver to the use of 30 ft/mb and 120 ft/C deg deviation when playing with pressure and density heights.

That's fine, providing that you keep in mind that these constants, in general, are wrong. For lower levels, they are adequate and fit for purpose. However, if you are playing with clearance considerations from the hard bits, the variation in pressure height calculation errors really need to be kept in mind.

A link to an explanation Bob Tait's Aviation Theory School - Altimetry Rates - Bob Tait's Aviation Theory School Forums
Originally Posted by FullWings
Another important point is that things like airfield and terrain elevation in aviation are measured in feet (in most places) and these feet are always the same absolute length. A “foot” on your altimeter is only on very rare occasions equal to a foot as you would measure it with a ruler, due to the fact that pressure and temperature have a role to play.

Great for separating aircraft on the same pressure setting from each other, but not so great at separating aircraft from terrain which is why there are safety margins built in, plus corrections for atmospheric conditions significantly below ISA.
Very useful in case they ask further questions, thanks !

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