Technology, New and Old.
A recent AINonline article1 by Robert P. Mark makes reference to the Dutch Safety Board’s desire to “publicize the existence of false glideslope indications that could cause the aircraft, when coupled to the autopilot, to pitch up rather than down.” Links to this article and the Dutch press release2 on this flight safety concern are provided in the footnotes below.
Instrument pilots know that ILS signals can have lobes that may provide faulty glideslope indications. A 3 degree signal may also generate spurious lobes in multiples of 3 degrees, most significantly 6 and 9 degrees. If approached from above, the ILS may lock on to one of these false signals. Pilots may know, but autopilots do not as the Dutch safety bulletin testifies. In addition, in some circumstances the signals may be reversed—and this particular incident is one example—and autopilot pitch up may be commanded. Is unwavering faith in your automation justified? Trust but verify.
Tried and true lessons to be learned/re-learned: Approach the glideslope from below; Verify your position, altitude and descent rate at key checkpoints.
Thomas P. Turner, a contributor to this forum, writes a newsletter, FLYING LESSONS weekly, and covered the Dutch incident and the false glideslope phenomenon in a recent edition3. Thomas is a highly experienced CFII and author and provides detailed visuals, explanations and examples that served as an inspiration for this Cockpit Concepts. You are able to subscribe to FLYING LESSONS weekly on his website, http://www.mastery-flight-training.com, and I encourage this readership to do so and take advantage of Thomas’ expertise, particularly with respect to the full range of general aviation aircraft.
--Bob Jenney (firstname.lastname@example.org)
1AINonline, Dutch Report on False Glideslope Signals, Robert P. Mark, July 14, 2014. Go to: http://www.ainonline.com/aviation-news/ainsafety/2014-07-14/dutch-report-false-glideslope-signals.
2Press Release, Dangerous autopilot response due to false glide slope, Dutch Safety Board, June 26, 2014. Go to:http://onderzoeksraad.nl/uploads/fm/PB_stickshaker_en_ILS_-_EN_DEF.pdf.
3FLYING LESSONS weekly, Thomas P. Turner, August 21, 2014. Go to: http://www.mastery-flight-training.com/20140821flying-lessons.pdf.