Page 6 - March 2016

HOW LOW
did you go?
The Bell 212 crew had waited in vain for seven hours, hoping the weather would improve at the departure airport, Alerk Island, NWT, and the destination, an oil rig in the Beaufort Sea. The helicopter was finally dispatched in night IFR conditions to fulfill the leasing company’s requirement to have a machine and crew located at the drill site.
Once in range of the rig, the crew commenced an improvised IFR approach, in ice fog, descending to 500 ft MSL, an altitude believed to assure sufficient separation from the platform and sea below. The weather at the time was: wind east at 10 knots, visibility 1 1⁄2 in fog, 400 ft overcast, temperature -19C.
As the platform passed beneath them, the crew saw the glow from the rig and proceeded briefly outbound, then turned back in the direction of the lights, using a tear-drop maneuver while descending to a target altitude of 150 ft for the inbound leg. Slightly after reaching that altitude, and without warning, the helicopter struck the sea ice. It disintegrated upon impact; a post-impact fire completely consumed the aircraft. One of the two crew members suffered serious injuries.
There are a number of troubling aspects to the way this flight was conducted, including improper in-flight decision making, improper IFR procedures, a poorly planned (improvised) approach, and failure to go missed once visual reference was lost. But the Canadian Safety Board listed two primary causes in its report that speaks to the inherent dangers present when subtle errors creep into the aircraft’s altimetry. In the case of this Bell 212, the pilot’s altimeter, in compliance with regulations,
4 • TWIN & TURBINE
read 50 feet low when set with the proper barometric pressure. Still, it was the crew’s failure to accomplish the proper temperature correction that likely doomed the flight.
To understand how this could happen, it is useful to review the recently-updated FAA-issued temperature correction chart. While temperature correction hasn’t been mandatory in the United States for designated airports until recently, it has been understood for a long time that very low (below ISA) temperatures can have a dramatic effect on pressure altimeter accuracy. Knowing the temperature measured by the rig operators was -19C, at 150 feet the crew should have added 30 feet to their
by Adam Alpert
MARCH 2016