flyingturtle From Switzerland, joined Oct 2011, 4553 posts, RR: 18
Reply 1, posted (3 years 6 months 3 weeks 5 days 9 hours ago) and read 4530 times:
Altimeters should be pretty reliable... except some wasps decide to occupy the static port.
Does your question relate to pressure altitude? Temperature differences can cause wrong altimeter indications. Or do you mean the difference between QNH (standardized altimeter setting to maintain vertical separation between aircraft) and QFE (the correct altimeter setting in regard to the ground)?
Keeping calm is terrorism against those who want to live in fear.
Jetlagged From United Kingdom, joined Jan 2005, 2620 posts, RR: 25
Reply 2, posted (3 years 6 months 2 weeks 10 hours ago) and read 4530 times:
Pressure error is the difference between sensed air pressure (static or pitot) and freestream pressure (static or total) . This occurs because the local airflow affects the pressure around the port. In relation to an altimeter it means that the sensed static pressure will give an incorrect altitude reading unless it is corrected. Air Data Computers have correction tables based on flight test data which apply a correction to the sensed pressure to give a corrected pressure, from which the indicated altitude is derived.
On some aircraft if you select alternate static source in flight you get an uncorrected pressure measurement (or perhaps a less sophisticated correction) which means the indicated altitude jumps to a new value.
The glass isn't half empty, or half full, it's twice as big as it needs to be.