Threat
For the aviation community Large Height Deviations (LHD) events pose the worst credible outcome of Mid-Air Collision (MAC). Whilst mitigations were introduced to reduce the outcome, such as Airborne Collision Alerting Systems (ACAS) and Short-Term Conflict Alerts (STCA) the risk is still, nevertheless, present. Examining and managing pre-cursor events is key to continued safety improvements and provides measures to assess the strength and effectiveness of Operator controls. LHDs are occurring on a global level, with significant numbers of events in the Americas and Africa regions, the reasons for such events are numerous, but could include:
Identified Hazards
Data available indicates that the most common causes of LHD are related to Pilot or ATCO human factors, which may include errors in the communication of flight levels, the allocation of an incorrect level, or issues in the maintenance, arrival or departure of an assigned flight level. It is these most common error types which should be considered by industry SMS to ensure that appropriate safeguards and checks are in place to mitigate the threat of LHDs.
So where does RVSM fit into the LHD and associated risk picture? The RVSM approval status of aircraft is tracked as part of the LHD mitigation strategy, aircraft operating between Flight Level (FL) 290 and FL 410 are subject to reduced separation criteria and therefore should be RVSM approved to operate under such conditions. The Regional Monitoring Agencies (RMA) are responsible for monitoring and tracking RVSM approval status and for the height monitoring functions, which are conducted through Height Monitoring Units (HMU). Operators are responsible for ensuring that the aircraft in their fleet which are RVSM approved are height monitored at regular intervals (ICAO ANNEX 6 Part1). Failure to comply with the height monitoring conditions could increase the risk of LHD and from the Operator perspective could lead to a withdrawal of the RVSM approval. To ensure that continued improvements in safety are realised, positive reporting of LHD events must improve, this will enable the accurate assessment of “hot spots” and build awareness for the aviation community.
|
LHD CODE |
TLHD Code Description |
|---|---|
| A | Fail to climb/descend the aircraft as cleared. |
| B | Climb/descent without ATC clearance. |
| C | Incorrect operation or interpretation of airborne equipment (e.g., incorrect operation of fully functional FMS, incorrect transcription of ATC clearance or re-clearance, flight plan followed rather than ATC clearance, original clearance followed instead of re-clearance, etc.) |
| D | ATC system loop error (e.g., ATC issues incorrect clearance or flight crew misunderstands clearance messages) |
| E | Coordination errors in the ATC to ATC transfer or control responsibility as a result of human factors issues (e.g., late or non-existent coordination, incorrect time estimate/actual, flight level, ATS route, etc., not in accordance with agreed parameters) |
| F | Coordination errors in the ATC to ATC transfer or control responsibility as a result of equipment outage or technical issues. |
| G | Deviation due to aircraft contingency event leading to sudden inability to maintain assigned flight level (e.g., pressurisation failure, engine failure) |
| H | Deviation due to airborne equipment failure leading to unintentional or undetected change of flight level |
| I | Deviation due to turbulence or other weather related cause |
| J | Deviation due to TCAS resolution advisory, flight crew correctly following the resolution advisory |
| K | Deviation due to TCAS resolution advisory, flight crew incorrectly following the resolution advisory. |
| L | An aircraft being provided with RVSM separation is not RVSM approved (e.g., flight plan indicating RVSM approval but aircraft not approved, ATC misinterpretation of flight plan) |
| M | Other – this includes situations of flights operating (including climbing/descending) in airspace where flight crews are unable to establish normal air-ground communications with the responsible ATS unit. |
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