Tuesday, July 17, 2018
Transport Canada - Aviation Safety Letter Understanding Altitude Deviations

Understanding Altitude Deviations

by Ann Lindeis, Manager, Safety Management Planning and Analysis, Operational Support, NAV CANADA


Altitude deviations1 are serious events which, if undetected, can lead to losses of separation and the potential for collision with both aircraft and terrain. Figure 1 shows the altitudes where deviations were reported through NAV CANADA’s aviation occurrence reporting (AOR) system for the last two years for which complete data is available. The figure is broken down by altitude and shows, not surprisingly, that most altitude deviations take place in the lower altitudes, where aircraft are involved in making step climbs and descents.

Figure 1
Figure 1

Click on image above to enlarge.

Figure 2 shows the number of altitude deviations reported over the last two years, broken down by flight information region (FIR), and demonstrates that this issue is pertinent across Canada.

Figure 2

Figure 2
Click on image above to enlarge.

These data were presented and discussed at NAV CANADA safety forums held recently in Toronto, Ont., and Vancouver, B.C. This initiative was described in a previous Aviation Safety Letter (ASL) article (issue 3/2007), and provides an opportunity for NAV CANADA to discuss specific safety issues with customers. The discussions led to a clear understanding that altitude deviations are a concern to both operators and NAV CANADA, and that decreasing the safety risk they represent will require an integrated approach. Some of the potential contributing factors leading to altitude deviations, which were discussed, include:

  • The challenges of complying with late descent clearances in modern, highly-automated aircraft when the aircraft is relatively high and close to the airport;
  • The increased numbers of altitude clearances received when aircraft are vectored off the standard terminal arrival (STAR);
  • The fact that most altitude clearances come in the terminal environment when the crew are in a period of high workload;
  • The potential for communication problems in receiving altitude clearances (see related article in ASL 2/2008 on communication errors).

If you or your organization are interested in working with NAV CANADA to better understand and mitigate the problem of altitude deviations, please contact Ann Lindeis at This e-mail address is being protected from spambots. You need JavaScript enabled to view it or 613-563-7626.

1 Altitude deviations include events where the aircraft deviated from an assigned or designated altitude. This may include deviations due to turbulence or other weather events. Flights may be conducted under instrument flight rules (IFR) or visual flight rules (VFR). For the purposes of this analysis, these do not include standard instrument departure (SID) deviations, as these are analyzed separately.


This article was published by Transport Canada in TP 185 Issue 3/2008 -. Reprinted with permission

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