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 !