A while back we discussed blank avionics screens and how dependent we’ve become on the hardware (Nothing can go wrong…). There was a recent Aviation Safety Reporting System ( NASA ASRS) report on a wild and crazy autopilot (AP) that went berserk on a Piper Mailbu.
“WE HAD JUST STARTED TO RECEIVE VECTORS FOR THE ILS/DME RWY X APCH INTO ZZZ. THE AUTOPLT SUDDENLY PITCHED THE ACFT NOSE DOWN ABOUT 30 DEGS. WE WENT THROUGH OUR ASSIGNED ALT OF 3000 FT DOWN TO ABOUT 2400 FT. (AIRSPD WAS AROUND 165 KTS.) BEFORE I COULD GET THE AUTOPLT DISENGAGED, THE PLANE SUDDENLY PITCHED UP VIOLENTLY AND THEN PITCHED DOWN AGAIN WITH ENOUGH FORCE THAT EVERYTHING WAS FLYING AROUND THE COCKPIT AND CABIN.”
In a subsequent discussion with the pilot, he said he attempted to manually over ride the autopilot before hitting the red disengage button on the control yoke. Rule number one on APs is to know at least three ways to retake command. This will be by yoke switch, panel switch, and circuit breaker – at a minimum.
To put it in political terms, APs are extremely partisan – they will always trim against you when engaged in a tug of war. For every pound of force you apply, they will counter. When you finally cut it off, the aircraft will be totally out of trim, and if you’re holding the barely equal and opposite force to balance, it will take strength, time ,and altitude to get things back to equilibrium.
I’m a big believer in APs and that they are essential for single pilot flight in significant IMC. But as with the glass, we need to be ready and able to discipline an unruly crew member and fly to a safe landing.