Flying tips: When flight instruments lie
Saturday, 11 February 2012 14:00

Flight -instrumentsDuring our early training days the instructor drilled it into our heads that, absent any visual reference to the outside world, our inner ear would soon fool our minds into believing that the airplane was pointed one way when in truth something entirely different was occurring.

In these instances, left could become right and up could become upside down. How to overcome the dangers of spatial disorientation when flying in clouds or on a dark night? Trust the flight instruments, we were told.

That’s excellent advice – almost all of the time. On takeoff at night in an airplane equipped with a gyro-based attitude indicator, a quirk of physics similar to the one that makes the magnetic compass tell lies (known as acceleration error) will also cause the AI to show a higher attitude than actual. As a consequence, some pilots have lowered the nose to a “normal” attitude on takeoff at night only to settle back to terra firma, sometimes with disastrous consequences.

The moment an airplane rotates and lifts off the runway on a dark night, it is enveloped in blackness. Outside visual reference becomes impossible. Making matters worse, somatogravic head-up illusion (involving a sudden linear forward acceleration where we perceive that the nose of the aircraft is pitching up) causes an instantaneous reaction in which we want to push the yoke forward to pitch down. A night takeoff from a well-lit airport into a totally dark sky (or a catapult takeoff from an aircraft carrier) can create this illusion.

Because the attitude indicator is the only instrument in the six-pack panel that reacts instantaneously to pitch and roll inputs, being unable to put your utmost trust it in at a critical phase of flight – low and slow at night – can be worrying. But as long as you understand acceleration error and don’t fall prey to its seductive powers, it’s really no problem. The proper technique is to fly the attitude indicator while relying on the airspeed indicator for reliable pitch information.

The next time you fly during the day, glance at the attitude indicator as you apply takeoff power. You will notice that the AI horizon line dips down, giving the misleading impression that the climb attitude is too steep. If you know what to expect from the AI on rotation at night and learn to trust your airspeed indicator, climbing out at the proper flight attitude will become second nature.

(Flying Magazine)

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