Large Height Deviation (LHD)

Understanding A Large Height Deviation (LHD)

The formal definition of a LHD is the following (ref. RMACG/12):

"A vertical deviation from an ATC assigned or coordinated altitude that results in an error of 300 ft or more. The deviation may be the result of human error, equipment malfunction or environmental factors such as turbulence, and should be reported in accordance with the LHD types defined here."

Essentially, a LHD happens when an aircraft occupies space unexpected by the ATC, leading the trajectory anticipation made by the ATC to no longer corresponds to the reality of an aircraft trajectory. Not knowing that the space is occupied, the ATC may clear another aircraft to that location, which may cause a mid-air collision.

Why is it important to report LHDs?

  • RMAs use LHD data to estimate airspace risk. The most important parameter is LHD duration. Roughly speaking, the longer the duration, the higher the risk.
  • The resulting risk estimates act as a trigger for States to initiate collaboration to solve safety issues, especially when the risk exceeds the Target Level of Safety (TLS) of 5.0 x 10-9 Fatal Accidents per Flight Hour (FAPFH).
  • RMAs provide annual reports to the Regional Airspace Safety Monitoring Group (RASMAG), which reports to APANPIRG. States can also use information provided in the report to help identify their safety issues.
  • You can use our presentation about LHD to promote LHD reporting to air traffic controllers. Please contact us if you would like the editable version.

Important Notes

  • Each ATS authority is responsible for reporting LHD to the responsible RMA (see Responsibilities of States).
  • An LHD contributes to the risk regardless of whether a loss of separation occurred or not.
  • Detailed description of large height deviation occurrences is crucial for the RMA to assess the risk of the LHD and its duration.
  • Each ATS authority should have an internal safety management system that defines an internal reporting process and the treatment of each report. The system should take into account the LHD reporting requirement.
  • Please read more about cross-boundary LHDs here.

LHD Examples

Here're some examples of LHDs. More detailed categories and examples of LHDs can be found here. Frequently-asked-questions about LHDs can also be found here.

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How to Report LHDs

Typically, the POC person in each ATS Authority collects LHDs during each month and sends the reports to the respective RMA.

  • The list of LHD POCs is distributed via e-mail. Please contact us if you have not received the list.
  • New LHD report form can be found here.
  • LHD reports are to be submitted monthly. We encourage reporting units to use the subject line of the email submission as follows:
    • "[FIR's name] LHD Reports for [Month] [Year]", for example, Bangkok LHD Reports for Dec 2016.
    • For NIL report for the month, simply write "[FIR's name] LHD Reports for [Month] [Year] = NIL" (no file attachment is required)
  • Please note that MAAR is working on a prototype system to enable online submission & automatic notification.