Monitoring height keeping performance is to estimate and evaluate the Altimetry System Error (ASE) of an aircraft operating in the RVSM airspace. The ASE is the difference between the altitude indicated by the altimeter display, assuming a correct altimeter barometric setting, and the pressure altitude corresponding to the undisturbed ambient pressure. In more simple terms, the ASE is the difference between the altitude which the pilot, ground controller, and aircraft systems believe aircraft to be at and the actual altitude. To be compliant with international standards, the ASE of an aircraft must be less than 245 ft.

The number of aircraft required to be monitored is dependent on the aircraft type specified in Asia/Pacific Minimum Monitoring Requirements (MMR). Furthermore, evidence shows that the ASE characteristics are not constant but drifting in a linear fashion over time (see Figure below). Therefore, ICAO has put in place the standard and recommended practice of the long-term monitoring requirements (Annex 6 Part I Section 7.2.7 and Part II Section 2.5.2.7), to ensure the safety of flight operations within the RVSM airspace:

“The State of Registry that has issued an RVSM approval to an owner/operator shall establish a requirement which ensures that a minimum of two aeroplanes of each aircraft type grouping of the owner/operator have their height-keeping performance monitored, at least once every two years or within intervals of 1 000 flight hours per aeroplane, whichever period is longer. If an owner/operator aircraft type grouping consists of a single aeroplane, monitoring of that aeroplane shall be accomplished within the specified period.”

Example of Single Airframe Altimetry System Error Degradation over 5 years

Summary of HKP monitoring requirements:

• Only aircraft with RVSM airworthiness approval may conduct the HKP monitoring;
• Operators seeking/maintaining RVSM approval are required to have a number of their aircraft monitored the HKP within 6 months of receiving the RVSM operational approval;
• Number of aircraft required to be monitored is dependent on the aircraft type specified in Asia/Pacific MMR ;
• Re-monitoring of aircraft is required based on the ICAO long-term monitoring requirement;
• To ensure collection of sufficient monitoring data, the aircraft must fly at a single flight level between FL 290 and FL 410, and should be at least 40 minutes in duration.
• The HKP monitoring should be conducted in areas where Mode-C radar data is retrievable in a timely manner. If the HKP monitoring is conducted outside the Mode-C radar coverage area, the operator shall provide pressure altitude data from the Digital Flight Data Recorder (DFDR) to the height keeping performance monitoring service provider.
• The measured ASE of an aircraft must be less than 245 ft.
• Monitoring data from any regional monitoring programme established in accordance with Annex 11, 3.3.5.2, maybe used to satisfy the requirement.