Altimeter and VSI opposing indications
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Join Date: Dec 2005
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Altimeter and VSI opposing indications
OK guys I am not an airline pilot but many years ago I was flying on the cockpit of an airliner, over the Atlantic Ocean.
We were at 30 (or 31),000ft and got clearance to climb to 32,000ft. The aircraft was put into a shallow climb on autopilot at about 400fpm and something odd happened, the vertical speed indicator showed a climb of above number, but the altimeter slowly started winding backwards (downwards), over the next 5-10 seconds, to about 50ft below the initial cruise altitude. The Captain entered a higher VS number of 700fpm into the autopilot and the altimeter then showed the climb normally.
It seemed odd it was showing a climb (and the pitch angle had gone up slightly) but the altimeter showed a brief descent at the same time. Was this some kind of instrument inaccuracy or something to do with the way they work? It happened only about 10 seconds then went back to normal. I'm remembering it right because the Captain pointed it out as well.
It wasn't very turbulent I remember, but there were quite a few clouds around near our level and strong upper level winds.
We were at 30 (or 31),000ft and got clearance to climb to 32,000ft. The aircraft was put into a shallow climb on autopilot at about 400fpm and something odd happened, the vertical speed indicator showed a climb of above number, but the altimeter slowly started winding backwards (downwards), over the next 5-10 seconds, to about 50ft below the initial cruise altitude. The Captain entered a higher VS number of 700fpm into the autopilot and the altimeter then showed the climb normally.
It seemed odd it was showing a climb (and the pitch angle had gone up slightly) but the altimeter showed a brief descent at the same time. Was this some kind of instrument inaccuracy or something to do with the way they work? It happened only about 10 seconds then went back to normal. I'm remembering it right because the Captain pointed it out as well.
It wasn't very turbulent I remember, but there were quite a few clouds around near our level and strong upper level winds.
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I suggest that the aircraft had just encountered a shear gradient and flown into warmer air, so despite being at the same physical altitude the altimeter would indicate slightly lower due to the slightly higher static pressure. However the Inertial part of the IVSI would show a climb initially (if the aircraft trajectory was physically climbed at 400 ft / min.)
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Sounds like a static source error to me.
Difficult to say for sure as you've given absolutely no details of the aircraft type. (many modern airliners have correction for this using AoA, airspeed, pitch, yaw, etc).
The initial pitch of the aircraft would create a small additional positive pressure on the lower part of the fuselage and if the static ports were in the region of this pressure increase it would be indicated on the altimeters (small descent)
The dashpot within the VSI (or the inertial input from the IRS) means it would however correctly indicate a climb.
Difficult to say for sure as you've given absolutely no details of the aircraft type. (many modern airliners have correction for this using AoA, airspeed, pitch, yaw, etc).
The initial pitch of the aircraft would create a small additional positive pressure on the lower part of the fuselage and if the static ports were in the region of this pressure increase it would be indicated on the altimeters (small descent)
The dashpot within the VSI (or the inertial input from the IRS) means it would however correctly indicate a climb.
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Using a static-driven altimeter and VSI I can easily get them going opposite directions. I do this as a demo for students who spend too much time not looking out the windows 
Not sure about the IVSI, but a rate sensing instrument and a static air driven instrument are sensing different things and could very well get out of synch for a bit.
Not sure about the IVSI, but a rate sensing instrument and a static air driven instrument are sensing different things and could very well get out of synch for a bit.
