Aviation Safety Letter 1/2004

Collision with Windrow

Artist's impression of collision with windrow.
Artist's impression of collision with windrow.

On January 11, 2003, a Beech 1900D was taxiing on Runway 02/20 at St. John's International Airport, Nfld., when the aircraft struck a 2-ft high windrow, which lay across the runway just to the north of Taxiway Charlie. There were no injuries to the 10 passengers and 2 crew members. The aircraft sustained substantial damage. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A03A0002.

St. John's Airport has three runways - 11/29, 16/34, and 02/20 - and all three runways are used throughout the year. Because of its Category II instrument approach capability, and unless wind conditions clearly favour Runway 16/34, snow clearing efforts are initially focused on preparing Runway 11 and the associated runways and taxiways that are necessary for access. Runway 02/20, north of Taxiway Charlie to the entrance to Taxiway Bravo, is last in priority when either Runway 11/29 or for the closing of an active runway because of contamination. One of these criteria is that an active runway should be closed if there are windrows in excess of 12-in. high. The plan, however, does not contain instructions for the closing of non-active runways such as the portion of Runway 02/20 north of Taxiway Charlie.

Windrows in areas such as the one north of Taxiway Charlie are not normally encountered by taxiing aircraft, and are not usually reported to the ground controller, nor indicated on the runway surface condition (RSC) reports. The normal practice at the airport is for snow removal crews to advise the ground controller of windrow hazards when they hear a taxi clearance, or a request for taxi, through an area with a windrow. Canadian Aviation Regulation (CAR) 302.07(2)(b) Obligations of Operator, requires that the airport operator provide the air traffic control (ATC) unit with immediate notice of the existence of any obstruction or hazardous condition affecting aviation safety at the airport.

The night before the accident, the weather was varied. Light snow had been falling and maintenance activities were focused on clearing snow from Runway 11 and the related taxiways. It could not be determined precisely when, or by whom, the windrow was created; however it is during this initial period that the windrow to the north of Taxiway Charlie was created by plowing activities.

Between 06:00 and 06:30, freezing drizzle started falling at the airport and a taxiing Airbus slid while backtracking on Runway 11; the crew radioed the tower that they were at the runway edge and unable to manoeuvre from that position. Coincidentally the wind shifted, favouring Runway 16. The lead-hand and the field maintenance crews were now preoccupied with two tasks: extricating the Airbus from the button of Runway 11, and preparing and inspecting Runway 16 for use. By about 07:55, the Airbus had been towed and inspected by company maintenance personnel.

Around 08:15, the day lead hand proceeded onto the airfield to relieve the night lead hand. Normally, one of the first actions of an oncoming lead-hand is to inspect the entire airfield by vehicle. However, in this instance, the oncoming lead-hand proceeded directly to Runway 11/29 to meet with the night lead hand and lend assistance with the Airbus. The customary airfield survey was not completed. The wind shifted again and it was decided to switch to Runway 29. Airfield maintenance sanded the runway and the threshold of Runway 29. The night lead-hand completed an inspection of Runway 29, and then departed the airfield at about 08:22. The Airbus took off from Runway 29 at 08:40.

At 08:41, the Beech 1900D was cleared to taxi via Foxtrot, Runway 02, and to hold short of Runway 29. An analysis of the flight data recorder (FDR) information showed that the aircraft was travelling at about 8 kt and was accelerating when it rolled out on the runway, heading on Runway 02. As the aircraft approached the intersection of Runways 02 and 34 there were several snow removal vehicles on Runway 34 east of the intersection. After confirming that the vehicles were holding short, the crew proceeded through the intersection, initiating the "instruments" portion of the taxi check 9 seconds prior to striking the windrow. According to the operator's standard operating procedures (SOPs) for the instrument cross check, the captain (pilot flying), must look inside the cockpit to call out indications from the aircraft's flight instruments. Meanwhile, the first officer (pilot not flying) is also supposed to monitor the instruments and when found correct should respond, "Checked and set left/right." As the crew was conducting this check, the windrow was spotted. The aircraft was now 3.5 seconds and 146 ft from the windrow and travelling at 24 kt. The captain attempted to stop with wheel braking, approximately 2 seconds later and 60 ft short of the windrow. When the wheel brakes were applied, the aircraft started to skid on the slippery runway surface. The captain attempted to apply reverse thrust, but there was insufficient time for it to be selected before the nose wheel struck the windrow, at 23.5 kt. The propellers struck the windrow next, followed ¼ second later by the main gear, which struck the windrow at approximately 20 kt.

The lowest point of the propeller tip path on the Beech 1900D is 14.07 in. from the ground. When the propeller blades struck the 2-ft high windrow, all 4 blades from the right engine and 1 blade from the left engine broke off near the hub. The blades from the right engine struck the starboard aircraft fuselage at the forward passenger cabin window. This window shattered and the window fragments and frame were thrown forcibly into the cabin. A mother and her infant, who were seated immediately next to the window, narrowly escaped injury. The crew stopped the aircraft 175 ft past the windrow, secured the engines, shut off the electrical power, and escorted the passengers from the aircraft. The ground controller noticed the passengers deplaning and activated the crash alarm.

Runway 02, from Taxiway Charlie to Runway 29, had not been traversed by any vehicle prior to the accident. The runway had not been cleared during the previous night, and there were no RSC reports produced for the runway. As the location of the windrow north of Taxiway Charlie was well to the north of Runway 16/34, the windrow did not appear on any of the RSC reports produced that night. After the accident, Runway 02/20 north of the windrow was found to be covered with a combination of ice and patches of thin snow.

Analysis - Several factors combined to allow this large windrow to remain unreported. Neither lead-hand nor the ground controller were made aware of the creation of the windrow because of the practice of only reporting windrows on active runways. The location of the windrow was in an area that was not used by either ground vehicles or aircraft until Runway 29 became active, and it was outside of the areas inspected by the night lead-hand during his shift. The night shift had a significantly increased workload because of the freezing drizzle, the stranded Airbus, and the frequent runway changes. These factors likely diverted attention away from ensuring that the taxi route north of the Charlie intersection was usable when Runway 29 became active. The shift change for the snow removal crews coincided with the towing of the Airbus, the runway change, and the issuing of the taxi clearance to the Beech 1900D. The windrow was the result of snow plowing activities, and it is likely that whoever had knowledge of the windrow had departed the field prior to the taxi clearance being issued and would not have been available to warn of the existence of the windrow. The oncoming lead-hand did not perform the customary field survey and inspection because of the pressing need to prepare Runway 29 and move the stranded Airbus. A field inspection would have allowed for the detection of the windrow and for action to remove it or communicate its presence to the ground controller.

Runway 02 north of Charlie is a low priority surface, and was not used prior to the Beech 1900D by either vehicles or aircraft. The surface had not been cleared, was not usable, and was not necessary, yet it remained open. The St. John's airport winter maintenance plan does not contain guidance to field maintenance personnel for the closure and subsequent re-opening of these non-essential surfaces.

The crew's previous safe transit through the intersection and the lack of any warning of obstructions along their taxi route resulted in them proceeding with their normal taxi routine, and without extra vigilance for taxi hazards such as windrows. The flat-light conditions and the white background of the uncleared portions of the airfield also caused the windrow to blend into the background, making it less conspicuous from a distance. Approaching the intersection, the crew's attention was diverted by the presence of snow removal vehicles on Runway 16/34 that were approaching their location. During the subsequent taxi check, the first officer was reading the checklist and the captain's attention was focused inside the cockpit, as he was verifying his flight instruments. These actions and the inconspicuousness of the windrow prevented the crew from seeing it earlier.

The taxi speed of the aircraft and the icy condition of the runway hindered the stopping of the aircraft, and consequently did not allow time for the captain to apply reverse thrust. Had the aircraft's taxi speed been less, more time would have been available for the crew to recognize and react to the windrow. With more time to react, it is possible that the crew could have stopped the aircraft prior to the collision.

As a result of this occurrence, the St. John's Airport Authority has issued a memorandum, which allows the lead hand to close Runway 02/20 when conditions require.

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