Aerodrome Safety Circular (ASC) No. 2002-014

ASC 2002-014


Operation and Modification Options for Precision Approach Path Indicator (PAPI) Units.


This Circular supersedes Aerodrome Safety Circular (ASC) 98-002, Operation of Precision Approach Path Indicator (PAPI) Units, dated 1998.07.24, which is hereby revoked, effective immediately.


This ASC is to advise aerodrome operators of Transport Canada's reviewof current information related to PAPI units producing false signals resulting from 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 Pacific Region Aerodrome Safety office was also made aware of a similar situation regarding false PAPI signals caused by frost contamination on the PAPI lens.

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 were to be (1) kept on continuously at the specified current level, for sites having aircraft radio control of aerodrome lighting (ARCAL) systems, and (2) turned on a half-hour before flight arrival at sites having air traffic service (ATS) control in order to maintain signal integrity and meet regulatory requirements.

The test program used PAPI units from three manufacturers/suppliers, Siemens, Alstom(Cegelec) and Crouse-Hinds, and concluded the following:

  • Contaminants, such as ice, dew or frost, on the PAPI front lens surface do affect the projected signal.
  • If contaminants exist on the PAPIs and the units are operated at their maximum current setting of 6.6 amp., approximately a half an hour is required to remove contaminants to the point where a true signal is produced at temperatures as low as -30° Celsius.
  • Where continuous operation was used, it was found that providing a minimum current of 4.8 amp. to the PAPIs was sufficient to keep the lens/cover glass free of contaminants that 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 develop a satisfactory solution to this problem. These results and subsequent conclusions led to the issuance of ASC 98-002.

Recent Test Developments

  1. Results of the recent tests conducted by PMG Technologies in Blainville, Que., for Transport Canada on specific PAPI design modifications indicated that it is possible to retrofit existing PAPI units with circuits consisting of lens heaters and monitoring thermoswitches. The modification used the existing PAPI circuitry and the available field power. These tests showed that frost and dew contaminants can be cleared from the lens within five minutes of activation. During the clearing time period, the PAPI lights are not energized, and no signal is projected while the electrical power is directed into the heating circuit that clears the lenses. At the end of the clearing cycle, the heater circuit is de-activated, and the lights are energized to produce a true signal.
  2. Information provided by Crouse-Hinds Airport Lighting Products, a division of Cooper Industries (Canada) Inc., recommends the installation of a selection relay. This will operate the PAPI/abbreviated PAPI (APAPI) continuously at 50% of the rated power when idle, providing 3.3 amp., 50 W during daylight and 2.12 amp., 20.7 W during darkness to each light. Their information indicates that these power levels prevent frost from forming on the lenses over the operating range of the equipment of +50°C to -50°C. The ARCAL, on receipt of the appropriate signal, will immediately cause the current selection relay to power the PAPI/APAPI at their rated current (4.8 to 6.6 amperes) until time out.

Guiedlines for PAPI Modifications

Based on background information and the recent new test developments, Transport Canada encourages aerodrome operators with PAPI units to consider taking the following action:

in collaboration with the PAPI manufacturer, implement the most efficient and cost-effective solution that will ensure the PAPI/APAPI units operate in such a manner that a false signal will not be projected.

Possible solutions include the following:

  1. Employ a modified circuit endorsed by the manufacturer that clears the PAPI lenses of contaminants within five minutes of activation. A signal will not be projected during the clearing time.
  2. Employ a modification endorsed by the manufacturer that will facilitate operation at an appropriate current/voltage selection that will ensure lenses are maintained in a contaminant-free state.
  3. In consultation with the PAPI manufacturer, develop other alternatives that will facilitate an economical resolution to the lens-frosting problem.

Any modification(s) to PAPI units should not alter the performance requirements for safe operation of the PAPI units. The modifications should be such that the PAPI will project solely a true signal on activation.

Guidelines for Unmodified PAPI Units

Results from the earlier test on (unmodified) PAPI units indicated that care must be taken to ensure that PAPI units are operated in accordance with the standards set out in Chapter 5 of Aerodrome Standards and Recommended Practices (TP 312).

The following actions should be considered until proper modifications to PAPI units are implemented.

  1. At aerodromes having ARCAL, the PAPI should be operated continuously at a minimum current level of 4.8 amperes.
  2. At aerodromes having 24-hr ATS service, the PAPI should 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-hr ATS service, if there is a duration of several hours between the day's flights and those expected at night, the PAPI should again be operated at the maximum current level of 6.6 amperes for at least a half-hour before the arrival of the first flight.
  4. Where there is more than one PAPI at the aerodrome, these should be operated simultaneously in accordance with (1), (2) and (3) above.
  5. Where a PAPI is not producing a proper signal after the warm-up period, the PAPI is not in compliance and a NOTAM informing pilots that the PAPI is out of service should be issued.
  6. If the PAPI has to be used before completion of the warm-up period, the PAPI should be visually inspected and, if necessary, manually cleaned before each operation.
  7. Should the aerodrome operator not be able to accomplish any of the above, and if the PAPI is not in compliance, the PAPI should be taken out of service.

Airport operators are cautioned to ensure that PAPI units are operating in compliance with the associated regulations and standards.


This Circular is provided as an update and to summarize recent test and manufacturer information that appear to offer acceptable solutions to the problem of false signals resulting from frost and dew contaminants on PAPI lenses. However, Transport Canada strongly recommends that any modification to PAPI units should only be done in collaboration with, and endorsed by, the PAPI manufacturer. In addition, any PAPI undergoing modification should be thoroughly tested for its accuracy and integrity before being returned to service. PAPI units are to be installed and operated in accordance with the criteria set out in the appropriate regulations and standards. If the equipment does not meet these requirements, a NOTAM should be issued to inform pilots that the PAPI is out of service.

A list of regional Transport Canada Aerodrome Safety offices is attached.

These documents are available electronically at:

Original signed by:
John Maxwell (2002.03.26)
Director, Aerodrome Safety

Date modified: