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. LHD reports are to be submitted monthly, even if there are no report during that month.

• Since October 2019, we have been using the Online LHD Reporting System. The link to the system is available here. With the new system, users can view, edit and delete previously submitted LHDs.
• For States who still wish to submit the form via email:
  • Please use the Word document form here, then e-mail the file to us with the subject line "[FIR's name] LHD Reports for [Month] [Year]".
  • For NIL report for the month, simply write "[FIR's name] LHD Reports for [Month] [Year] = NIL" (no file attachment is required)

LHD Analysis and Mitigation

RASMAG emphasized the importance of States' involvement in solving LHD problems in term of analysis and action taken to reduce risk. The MAAR as a RMA propose following tools to conduct further investigation on LHD occurrences and encourage states to consider all possible mitigation means.

• LHD Analysis Form (Form A) : States are encouraged to conduct further investigation and provide in-depth analysis of LHDs, especially those induced by their responsible ATS units. The purpose is to understand the underlying causes in order to develop the appropriate mitigations
• LHD Preventive/Mitigation Measures Form (Form B) : States are encouraged to collect information on the actions that are being planned and already taken to prevent LHD occurrences or mitigate LHD risks, including the progresses of those actions.