A catalogue of errors
So, you think you know your altitude and your transponder will be giving reliable data? Read on! Pete Kynsey lists the precision errors that can so easily lead to an apparent infringement
1. Transponder error Transponders are tested only every year at most. The CAA requirement is plus or minus 125 feet. Checks (not true calibration, note-ed.) are done on the ground and do not therefore take into account errors generated in flight when the transponder is connected to the aircraft’s static system. Some manufacturer’s continued airworthiness requirements do not require any repetitive checks.
2. The aircraft’s static system This is never checked in the air for private aircraft so is an unquantifiable error. Commercial traffic operating in RVSM (reduced vertical separation) airspace are required to have extremely low static errors. However, evidence collected by the European Regional Monitoring Agency reveals that in some cases the error increased from fifty to 150 feet within twelve months, whereas it was previously thought to be constant. The error is likely to be far higher in NON-RVSM aircraft.
The transponder may not even be connected to the same static source as the altimeter. Some take their pressure from behind the panel, others from their own static source. Wherever it originates, the accuracy of the static pressure provided to the transponder at varying airspeeds and attitudes, including yaw, is never measured in GA aircraft. The transponder of an aircraft I recently flew passed the ground check, but was 340 feet in error in the air.
3. Altimeter error ICAO sets an international standard for Altimetry System Error (ASE) of 245 feet (ICAO document 9574). That means that the altimeter reading when set to 1013hpa can be up to 245 feet from the aircraft’s actual pressure altitude and still be acceptable. This error will be made up of altimeter error and static pressure error. The pilot is required to fly by the altimeter, not the transponder which may be sending out an entirely different figure.
4. No allowance for any the errors stated above is made by the Controlled Airspace Infringement Tool For it to convert the Flight Level it receives from your transponder into an altitude so that it can detect infringements where the base is defined as an altitude, CAIT must have a QNH set into it. If you are flying beneath CAS you are obliged to use an ‘appropriate QNH’, for example that of a nearby airfield. There is no guarantee that this is the same as the one CAIT is using, generating another error.
In a worst case scenario these errors can add up to 500 feet. Errors of half that are therefore likely to be common.