Problem: Flightlevel With Asa And Without

[Guest Mitcher]

Hi!Without ASA, the Aircraft is exactly on tuned Flightlevel, but with active ASA there is a difference about 300-400ft.Barometer is 1013 and FL350 is tuned.ASA activewithout ASAhow can i fix this problem?

[netshadoe 144]

Hi,There is no problem...that's what really happens. Your second picture shows a flight with local barometric pressure of 29.92in / 1013mb which will show exactly 35000ft on your altimeter and the red information bar at the top.In your first picture, with ASA running, the altimeter is showing your Flight Level which is FL350, but the red info bar at the top is showing what is known as your pressure altitude. This is because the local barometric pressure in the area is not 29.92in /1013mb. This is why the altitudes above the transition altitude are called Flight Levels because they don't truly represent the true altitude. Just as an experiment, just set your altimeter to your barometric pressure shown in ASA and you will see that the two will match.The reason we use Flight Levels is because over long distances, the pressure obviously does not remain constant. Pilots would constantly have to readjust their altimeters every 10 minutes to compensate for barometric change...not because they are worried to stay at 35000ft, but because if the altimeter is not set the same as other aircraft around them, a collision could occur. The 29.92/1013 setting puts everyone on the same pressure setting, regardless of what the barometer is reading on the ground. Two aircraft at FL350 will be at the same true altitude (which could for example be 35600ft). Over the flight, the aircrafts stays at FL350, but the true altitude will fluctuate up and down as it crosses barometric boundaries.Devin

[Guest Mitcher]

Devin, thx for this great answer !