IFR Fix: Nothing automatic about it

January 14, 2014

A corporate jet on approach suddenly pitches up and climbs several hundred feet above assigned altitude. Responsibility for the deviation lies with:  the crew, the autopilot, or an aircraft taxiing toward the runway?

If that seems like a knowledge-test gimme question, hold on. In the automation age, causes and effects of events like the one experienced by a Challenger CL604 in Teterboro, N.J., can be complex

And it turns out that the glitch, while automation-related, was rooted in aviation’s techno-past. When you studied ILS approaches, you read about the false glideslope—an aberration that can arise from a variety of causes including surface vehicles or another low-flying aircraft. 

The Challenger was on approach to Runway 19, in instrument conditions with the autopilot engaged, when "I noticed the glide slope indicator on the PFD rapidly move from full scale up to full scale down and then full scale back up again. The autopilot captured the false glideslope and the airplane abruptly pitched up from our captured altitude and began to rapidly climb in an attempt to join the glide slope from below," a crewmember reported to the Aviation Safety Reporting System.

The pilot disengaged the autopilot, but altitude had increased 300 to 400 feet. The pilot theorized that "antenna interference from airplanes as they taxi across Runway 19 on their way to Runway 24 for departure" may have caused the problem.

Does that jog the memory about known techno-hazards? Then here’s another question that actually appears on a sample instrument-rating knowledge test

"A generally recommended practice for autopilot usage during cruise flight in icing conditions is

A) having the autopilot continuously engaged while monitoring the system for abnormal trim, trim rate, or attitude.

B) periodically disengaging the autopilot and hand flying the airplane.

C) periodically disengaging and immediately reengaging the altitude hold function."

An FAA learning statement code directs you to study material that calls on you to recall the effects of icing on aircraft performance. Researching the issue takes you to an advisory circular that addresses the issue but does not conform exactly to the best multiple-choice answer. So take your best shot.

Even if there is no automation in your cockpit yet, your day may come. Stay ahead of the curve by including this subject matter in your proficiency regimen.

Which answer did you choose? Discuss in a comment below.

Dan Namowitz

Dan Namowitz | Aviation Writer

Dan Namowitz is an aviation writer and flight instructor. He has been a pilot since 1985 and an instructor since 1990.