Aerodrome Safety Circular

ASC 98-002


Operation of Precision Approach Path Indicator (PAPI) units


This Aerodrome Safety Advisory Circular is to advise aerodrome operators of Transport Canada’s investigation of PAPI units producing false signals due to the buildup of frost contamination on the front lens or cover-glass.


Concerns that false signals could be produced as a result of frost contaminated PAPI light units were first reported in 1996 by some airport operators in Quebec. In 1997, the regional office of aerodrome safety in Pacific Region was made aware of a similar situation regarding false PAPI signals at Kelowna airport due to frost contamination on the PAPI lens.

When Transport Canada investigated this issue it was believed the problem was limited to units installed at aerodromes in northern Quebec. As a result of its initial investigation, Transport Canada advised the affected aerodrome operators to keep their PAPIs operating continuously during the winter season.

Concerned about the safety hazard that this issue posed for aircraft operations into airports with PAPI units, Transport Canada undertook a study of the problem. In the meantime, airport operators with PAPI systems were informed through an urgent bulletin issued October 30, 1997 that until further notice PAPI light units are to be (1) kept on continuously at the specified current level, for sites having ARCAL control systems, and (2) turned on a half hour before flight arrival at sites having ATS control.

Testing Program

The test program used three types of PAPI units that are used at Canadian airports namely; Siemens, Cegelec and Crouse-Hinds. The objectives of the test program were to determine the following:

  1. Does contaminant on the PAPI lens surface affect the quality of the output signal?
  2. What is the time required to remove contaminant from the PAPI lens when frost build up has occurred?
  3. What are the PAPI’s abilities to prevent frost accumulation (contamination) during continuous operation at various temperatures?

The results of the testing program indicated that;

  • Contaminants such as ice, dew or frost on the PAPI front lens surface does affect the projected signal.
  • If contaminents existed on the PAPIs and the units were operated at their maximum current setting of 6.6 amperes, approximately ½ hour was required to remove contaminant at temperatures down to -30 degrees Celsius to the point where a true signal was produced.
  • Using continuous operation it was found that providing a minimum current of 4.8 amperes to the PAPIs was sufficient to keep the lens/cover-glass free of contaminant which would cause a false signal.
  • The testing concluded that false slope indication produced as a result of contamination on the lens is a design problem. It is the responsibility of PAPI manufacturers to come up with an satisfactory solution to this problem.

Advisory Information

Based on the test results, Transport Canada requests that aerodrome operators with PAPI units take the following action;

  1. At aerodromes having ARCAL, the PAPI shall be operated continuously at a minimum current level of 4.8 amperes.
  2. At aerodromes having 24 hour ATS service, the PAPI shall be operated at the maximum current level of 6.6 amperes (maximum brightness) for at least a half hour before the arrival of the first morning flight.
  3. At aerodromes having 24 hour ATS service, if there is a long duration of several hours between the day’s flights and that expected at night, the PAPI shall again be operated for a minimum of a half hour prior to the arrival of the first flight at the maximum current level of 6.6 amperes.
  4. Where there is more than one PAPI at the aerodrome, these shall be operated simultaneously in accordance with (1), (2) and (3) as above.
  5. Where a PAPI is not producing a proper signal after the warm up period, a NOTAM must be issued that the PAPI is out of service.
  6. If the PAPI has to be used before completion of the warming period, the PAPI shall be visually inspected for the absence of frost.
  7. Should the aerodrome not be able to accomplish any of the above, the PAPI shall be taken out of service.
  8. It is recommended that operators contact the manufacturers to advise them of the critical need to come up with a satisfactory solution. This is especially important since operating the PAPI continuously has a significant cost impact.

Airport operators are cautioned not to relax any of the above requirements until manufacturers produce an acceptable solution.

Original signed by:
Harvey Layden (1998.07.24)
Director, Aerodrome Safety

Date modified: