Sunday, October 22, 2017
Transport Canada - Aviation Safety Letter TSB Communications on Visual Glide Slope Indicator (VGSI) Issues

TSB Communications on Visual Glide Slope Indicator (VGSI) Issues

The following are two Aviation Safety Advisories recently submitted to Transport Canada (TC) by the Transportation Safety Board of Canada (TSB)

Background
On November 11, 2007, a Bombardier Global 5000 departed Hamilton, Ont. (CYHM), for Fox Harbour, N.S. (CFH4), with two crew members and eight passengers onboard. On approach to Runway 33, the crew followed the visual glide slope indications from an abbreviated precision approach path indicator (APAPI) to guide their descent. At 14:34 Atlantic Daylight Time (ADT), the aircraft touched down seven feet short of Runway 33 at Fox Harbour. The landing gear was damaged when it came in contact with the edge of the runway, and directional control was lost when the right main landing gear collapsed. The aircraft departed the right side of the runway and came to a stop 1 000 ft from the initial touchdown point. All occupants evacuated the aircraft. One crew member and one passenger suffered serious injuries, while the others suffered only minor injuries. The aircraft suffered major structural damage. The TSB investigation into this occurrence (A07A0134) is ongoing.

The Fox Harbour Runway 33 VGSI is an APAPI system and is designed for use by aircraft with eye-to-wheel height (EWH) of up to, but not including, 10 ft (3 m). The crew had flown into the Fox Harbour aerodrome on at least 80 occasions and were familiar with the runway environment. They had relied on the Runway 33 APAPI guidance in the past to complete approaches, normally touching down within the first 500 ft of runway. However, previous flights were with smaller aircraft, such as the Challenger CL604 with an EWH of 12.1 ft (3.7 m). The crew had little overall experience on the larger Global 5000 with an EWH of 17.2 ft (5.2 m) and it was only their third time landing this aircraft at Fox Harbour.

Flight crew awareness of VGSI system limitations
VGSI information can be found in many different publications used by operating flight crews, such as the Transport Canada Aeronautical Information Manual (TC AIM), as well as in the Canada Air Pilot General Pages (CAP GEN) and the Canada Flight Supplement (CFS), both published by NAV CANADA.

For example, the CAP GEN describes the different types of precision approach path indicator (PAPI) systems available. A table provided in the Approach Lights Legend section gives the following information:

P1 PAPI for aircraft with eye-to-wheel height up to 10 ft.

P2 PAPI for aircraft with eye-to-wheel height up to 25 ft.

P3 PAPI for aircraft with eye-to-wheel height up to 45 ft.

AP Abbreviated PAPI for aircraft with eye-to-wheel height up to 10 ft.

Following visual guidance from a visual approach slope indicator system not appropriate for the type of aircraft operated can result in an unsafe threshold crossing height. This is especially critical when operating to a runway not served by an electronic glide path, when visual illusions might be present, or at night. Flight crew knowledge of the limitations associated with the different types of visual approach slope indicator systems in use is therefore essential in order to assess the appropriateness of the system to the type of aircraft operated.

Many small community aerodromes across Canada are serviced by aircraft with EWH exceeding the limitations of the aerodrome’s visual approach slope indicator systems. Furthermore, compared to older aircraft, newer aircraft, such as the Global 5000, now have landing and take-off performance capabilities allowing them to operate using short runways. Those short runways are often equipped with visual approach slope indicator systems appropriate for aircraft with EWH of less than 10 ft. This situation increases the exposure to the risk of landing with reduced threshold crossing height safety margin.

Even though information related to VGSI systems is available in multiple publications, the investigation has determined that while pilots are aware that different systems are in use, they are not aware of their associated limitations, nor are they aware of the significance of following guidance from a system that is not appropriate to the aircraft type operated. For example, it is not critical for a small aircraft to follow visual guidance from a P2 or P3, as it would only provide a greater threshold crossing height; however, any aircraft with an EWH greater than 10 ft following visual guidance from a P1 or an AP would not be assured a safe threshold crossing height.

The TC Flight Instructor Guide—Aeroplane (TP 975E) lists the topic of VGSI as a teaching point under the night flying section. Although instructors cover the different types of equipment and their associated limitations, the emphasis is put on the significance of VGSI system indications to the pilots, without discussing the risks associated with following VGSI guidance not appropriate for an aircraft type. This limited emphasis results in pilots relying on VGSI guidance not suitable for some of the aircraft types they are operating. The investigation has determined that a RED/WHITE on-slope indication on approach would be perceived by pilots as a confirmation that they were on a safe flight path to landing. Without considerations for the type of VGSI system generating the visual guidance, following an on-slope indication could result in a large aircraft not having a safe threshold crossing height.

Furthermore, the only related topic addressed in TC flight crew examinations is the interpretation of the different visual indications provided by VGSI equipment. There are no questions with regards to the limitations of the different types of VGSI currently in use (PAPIs).

Due to flight crew limited knowledge of the different VGSI systems in operation and the significance of their limitations on the safety of flight operations, flight crews will continue to follow visual guidance that may not be appropriate for the aircraft type they are operating. Those flight crews will therefore not be assured safe threshold crossing height.

Therefore, TC may wish to review the pilot training requirements so that flight crews are made aware of VGSI limitations as well as its impact on the safety of flight operations for their aircraft type.

Availability of aircraft EWH information
VGSI system guidance is important when approaching a runway not served by an electronic glide path, when visual illusions might be present, or at night. However, knowledge of an aircraft’s EWH is necessary in order to assess whether a VGSI system is appropriate for the aircraft type being flown.

At the time of the above-mentioned occurrence, the crew was not aware of the EWH of either the Challenger CL604 or the Global 5000. The Global 5000 EWH was not published in the aircraft flight manual (AFM), or otherwise available to the crew. Although information relevant to the operation of an aircraft is usually published in the AFM, the investigation has determined that EWH information is generally not available in the AFM.

In the past, large aircraft performance characteristics precluded operations from short runways such as Fox Harbour’s 4 885-ft Runway 33. Modern large aircraft with better short field performance are now able to operate from shorter runways, where they are more likely to encounter VGSI designed for smaller aircraft. A large aircraft with an EWH greater than 10 ft following visual guidance from a VGSI designed for a smaller aircraft is not assured a safe threshold crossing height. Without EWH information, this situation increases exposure to the risk of landing with a reduced threshold crossing height safety margin.

On November 26, 2007, the TSB issued an Aviation Safety Information Letter to TC, informing them that the approach was flown with reference from an APAPI that was not designed for a Global 5000 with an EWH that, at the time, was suspected to be greater than 10 ft. TC’s response stated that EWH information is not normally stated in the AFM, nor is there a requirement to do so. TC also pointed out that, should an operator require this information, the type certificate holder can provide it to the operator on request. The investigation has determined that even the type certificate holder may not have this information readily available.

Because aircraft EWH is not available to pilots, crews may continue to conduct approaches with an aircraft mismatched to the VGSI system, increasing the risk of an unacceptable threshold crossing height safety margin.

Therefore, TC may wish to review the requirements to have aircraft EWH information available for use by flight crews in aircraft publications.

 

 

This article was published by Transport Canada in TP 185E -. Reprinted with permission

Add comment


Security code
Refresh