– Performance Standard for Aircraft Pressure Altimetry and Maintenance Requirements for Certain Aircraft Instruments
The primary method used by pilots to monitor and maintain a desired altitude is with a barometric altimeter. It works by measuring ambient air pressure (static pressure) and providing a display to the pilot in units of height. These instruments, whether they are aneroid based or solid state, need to be checked on a regular basis to ensure that what is being presented to the pilot is accurate. This is particularly critical during approach and departures that have terrain obstacles in close proximity.
AD/INST/8 Amdt 4 and AD/INST/9 Amdt 6 formerly required either:
- the pressure altimeter/s fitted to an aircraft are tested every two years in accordance with the nominated altimeter maintenance standard; or
- all aircraft instruments and instrument systems are tested in accordance with specified requirements every three years.
Following correspondence from industry regarding those altimeter setting procedures, a review of those procedures exposed an indirectly related anomaly in published material regarding altimeter maintenance standards.
Conflicting maintenance standards that are defined for aircraft operating under the Visual Flight Rules (VFR) and Instrument Flight Rules (IFR) effectively allowed aircraft with altimeters maintained to different standards to operate in the same airspace which, in the worst case scenario, could result in a loss of separation between the two aircraft.
With the introduction of Wide Area Multilateration systems into the Sydney metropolitan area as well as large areas of Tasmania it is essential that accurate altitude information is broadcast, via the Mode C transponder, for Air Traffic Control purposes. This would apply to any aircraft transmitting Mode C information regardless of operation under either the VFR or IFR.
An unintended consequence of allowing a choice of actions within the Airworthiness Directives (ADs) had been the interpretation that, with opting to do the pressure altimeter check, other aircraft instruments and instrument systems did not have to be maintained. This may have resulted in the accuracy of critical aircraft instruments degrading.
Contact: Charles Lenarcic, Project Leader
- NFRM 1101CS - Performance Standard for Aircraft Pressure Altimetry and Maintenance Requirements for Certain Aircraft Instruments
- ANNEX A - Legislative Changes: Amendments to CAOs 100.5, 108.56 and cancellation of AD/RAD/43, AD/INST/8 Amdt 4 and AD/INST/9 Amdt 6
- ANNEX B - Summary of Responses to NPRM 1101CS