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Transport Canada - Aviation Safety Letter EMS Helicopter Crew Caught by Black Hole Illusion Issue 2/2010

EMS Helicopter Crew Caught by Black Hole Illusion Issue 2/2010

On February 8, 2008, a Sikorsky S-76A MEDEVAC helicopter departed Sudbury, Ont., for Temagami, Ont., to meet a land ambulance. At approximately 22:02 Eastern Standard Time (EST), while on final approach to the Temagami Snake Lake Helipad in night visual meteorological conditions (VMC), the helicopter crashed in the forested area at the edge of the lake. The helicopter came to rest on its left side and was substantially damaged. Three of the four occupants received serious injuries and were transported to the hospital. This article is based on the Transportation Safety Board of Canada (TSB) Final Report A08O0029.

Image of an helicopter crashed in a forested area.

The entire region was experiencing localized light to moderate snowfall on the evening of the occurrence and it was uncertain as to whether the flight would be able to land in Temagami.

The captain was the pilot flying (PF) and was certified and qualified for the flight in accordance with existing regulations. He had approximately 3 107 hr total flying time and 2 267 hr on the Sikorsky S-76A. Records indicate that he had received all of the company’s required training, including night visual flight rules (VFR)/instrument flight rules (IFR) and controlled flight into terrain (CFIT) with specific training for black hole approaches (visual spatial disorientation). The captain had been to this location once in the past, on a day VFR flight.

The first officer was the pilot not flying (PNF) and was certified and qualified for the flight in accordance with existing regulations. The first officer was hired in July 2007, and had all the required training. He was fairly new to emergency medical services (EMS) operations and had never been to this location.

On the night of the occurrence, the helicopter departed Sudbury at approximately 21:40 EST on a short flight to the Snake Lake Helipad in the town of Temagami, located approximately 60 NM to the northeast. The helicopter climbed to 2 500 ft and proceeded to Temagami. Throughout the initial portion of the flight, the visibility was found to be no less than 4 to 5 SM and improved as the flight progressed. The flight was uneventful and both pilots spent most of the time discussing procedures and co-ordinating the patient pick-up with dispatch. During the last 1.5 min of the approach, the PF was explaining to the PNF what he was doing, step by step, and what to watch for during night approaches, including black hole illusions.

The Snake Lake Helipad is located on the northeast edge of town. According to the operator’s landing site directory for the Sudbury/Moosonee district, the Snake Lake Helipad is at a field elevation of 997 ft above sea level (ASL) and has a 100 by 100 ft asphalt-surfaced pad with retro-reflective cones around the perimeter and with lead-in cones at 220° magnetic (M) from the pad. Four of the perimeter cones can be equipped with e-flares to aid in visibility. These must be requested by the flight crew and are placed and activated by ground EMS personnel. They were not requested on the night of the occurrence.

The directory cautions of the following hazards:

  • wires under, along east and north sides of the approach/departure sector;

  • large hills south, east, and north of the site;

  • tower west and fire tower south of the site;

  • ball park east of helipad.

Additionally, there is a single house located beside the ball diamond, which has typical outside door entrance lights.

The helicopter approached the helipad from the southwest on a heading of approximately 048°M and entered the trees near the edge of the lake approximately 814 ft horizontally from the helipad.

The trees on the approach averaged 40 ft in height. The helicopter impacted trees that were located on the downward slope of the hill, at approximately 70 ft horizontally from the shore, where the height of the hill is approximately 10 ft higher than the helipad. As such, the average tree tops were approximately 50 ft higher than the helipad. The descent into the trees was near vertical with very little horizontal momentum and the nose of the helicopter came to rest approximately 15 ft from the shore. The helicopter’s rotor diameter was 44 ft and the damage to the trees was mostly within this diameter. The rotor blades were completely destroyed. During the descent, a tree passed through the left landing gear bay, the main battery, and continued through the engine deck and exhaust collector of the right engine. There was evidence of heat and scorching on the tree consistent with the heat of a running engine, but no post-crash fire.

Snake Lake Helipad
Snake Lake Helipad

Click on image to enlarge

A detailed examination of the helicopter revealed no discrepancies that would have affected its flying characteristics. No damage was found that would have prevented the engine from running.

The helicopter was equipped with an enhanced ground proximity warning system (EGPWS), dual Garmin GNS 530 global positioning system (GPS)/Navigation/ Communication units, a Latitude Technologies SkyNode satellite tracking system, and a cockpit voice recorder (CVR). These components were removed and analyzed. There were no operating abnormalities with the helicopter or engines prior to impact, and the helicopter was on the proper descent profile until it reached 500 ft above ground level (AGL) and 0.5 NM from the helipad, 21.5 s before impact. The PF perceived that the helicopter was too high and corrected accordingly. Simultaneously, the cockpit area microphone picked up the sound of the rotor RPM increasing slightly, then decreasing just prior to impact. The rotor RPM recording also confirmed an increase and decrease in rotor RPM just prior to impact. The PNF did not question the PF’s deviation from the proper descent profile, nor did he make any further speed or altitude calls after the deviation.

According to a study by the United States Air Force, titled Running Head: BLACK HOLE ILLUSION, spatial disorientation is defined by Gillingham as: “an erroneous sense of one’s position and motion relative to the plane of the earth’s surface.” The study also states:

Visual spatial disorientation (SD) is often cited as a contributor to aviation accidents. The black hole illusion (BHI), a specific type of featureless terrain illusion, is a leading type of visual SD experienced by pilots. A BHI environment refers not to the landing runway but the environment surrounding the runway and the lack of ecological cues for a pilot to proceed visually. The problem is that pilots, despite the lack of visual cues, confidently proceed with a visual approach. The featureless landing environment may induce a pilot into feeling steep (above the correct glide path) and over-estimate their perceived angle of descent (PAD) to the runway. Consequently, a pilot may initiate an unnecessary and aggressive descent resulting in an approach angle far too shallow (below the correct glide path to landing) to guarantee obstacle clearance.

There were no anomalies found with the helicopter that would have contributed to the accident. Therefore, this analysis focuses on the operation of the helicopter.

The Snake Lake Helipad is a classic black hole approach helipad. Temagami itself is a small community and the helipad is on the northeast edge of town. The approach is flown over the town and past all the lights with a relatively featureless landscape forward. The only visible lights are those of the house beside the ball diamond. On the terrain along the approach path, a small hill begins to rise approximately 2 430 ft horizontally from the helipad. The maximum rise is approximately 20 ft, which then gently slopes back down to the lake surface 723 ft horizontally from the helipad. The mature trees along the flight path would further increase the obstacle height another 40 ft. However, the steep approach angle of 8° into the landing site would have provided for adequate clearance above the trees to land safely.

The black hole approach requires diligent monitoring of the helicopter’s instruments. The flight crew followed most of the standard operating procedures (SOPs) during the approach and appropriate calls were made. In this case, the PNF was monitoring the airspeed, altitude and distance to the helipad. He relayed this information to the PF regularly. The PF, flying a visual approach, utilized the information from the PNF in addition to the visual cues for reference. However, the PF’s radar altimeter was not set to 150 ft as called for by the operations manual. This would have provided an additional cue to the flight crew that the helicopter was approaching the ground too soon during the descent into the helipad. Meanwhile, the helicopter was on a stabilized approach with the proper 8° descent profile, as required by the operations manual and the SOP.

During the 1.5 min of the approach, the PF’s attention was split between flying the approach and explaining why things were happening and what to watch for during a black hole approach. This likely distracted the pilots from the task at hand. In this case, the PF acknowledged a 0.5 NM and 500-ft call, an on-profile condition, but visually perceived that the helicopter was too high and, therefore, increased the rate of descent. This coincides with the increase in the rotor RPM—an indication that the collective is being lowered, decreasing the load on the rotor blades and increasing the descent rate. This was followed by a decrease in rotor RPM as the collective was raised, increasing the load on the rotor blades and decreasing the descent rate just prior to impact. At no time did the PNF question the PF’s deviation from the proper descent profile nor did he make any further speed or altitude calls after the deviation.

Based on the available information, a descent from 500 ft to impact in less then 21.5 s equates to a descent rate of more than 1 400 ft/min—well in excess of the recommended maximum descent rate of 750 ft/min. The increased descent rate caused the helicopter to descend into the trees before either crew member realized what was happening.

Findings as to causes and contributing factors

  1. The PF was likely affected by visual spatial disorientation and perceived the approach height of the helicopter to be too high. While correcting for this misconception, the helicopter descended into trees 814 ft short of the helipad.

  2. The pilots were likely distracted during the critical phase of the approach and did not identify that the helicopter had deviated from the intended approach profile and recommended descent rates.

Findings as to risk

  1. The right rear aft-facing paramedic seat lap belt attachment barrel nut was worn in the groove where the seat belt attaches, weakening the barrel nut’s structural integrity, thereby increasing the risk of failure.

  2. The helicopter crashed on its side, placing an abnormal side load on the right rear aft-facing paramedic seat lap belt attachment barrel nut, thereby causing it to fail.

Safety action taken
Following the occurrence, the supplemental type certificate (STC) holder for the EMS interior utilized in the S-76, issued Service Bulletin No. SB-EMS76-1. This service bulletin identified the affected helicopters and called for the replacement of the existing lap belt attachment barrel nut with a steel shackle. All affected helicopters have complied with the service bulletin.

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