A human brain slowly deprived of its normal oxygen supply, called hypoxia— especially if it happens to someone in command of an airplane–can create strange behavior the affected aviator may not even recognize. The FAA says the most common causes of hypoxia in pilots are: flying non-pressurized aircraft above 10,000 ft without supplemental oxygen, rapid decompression during flight, pressurization system malfunction, or oxygen system malfunction.
A gradual loss of oxygen is precisely what the Transportation Safety Board of Canada recently cited as the cause for the 2018 loss of a Piper Navajo near Mount Rae, Alberta. The airplane, in contact with ATC, departed controlled flight shortly after the pilot reported engine problems at approximately 13,500 feet. The pilot and a survey technician were the only people on board. Both died when the airplane struck nearby terrain. The TSB determined that, although a portable oxygen system was activated and available, the pilot of the unpressurized Navajo, an experienced ATP-rated aviator, did not continuously use it while the aircraft was above 13,000 feet ASL, as required by Canadian regulations. Hypoxia was the likely result.
Although hypoxia can cause pronounced performance degradations, its onset can sometimes be gradual, making it likely the pilot did not recognize the symptoms, and this might explain why he took no action to restore his oxygen supply, the TSB said. The investigation also found that if flight crews do not undergo practical hypoxia training, there is a risk they will not recognize its onset when flying above 13,000 feet without continuous use of supplemental oxygen.
The Navajo was not equipped with either a flight data or a cockpit voice recorder, nor were these required by regulation. The aircraft was, however, equipped with a flight data monitoring system including a camera that provided some information to help investigators better understand the factors that contributed to the accident.
Minute-By-Minute
On 01 August 2018, after completing two hours of aerial survey work near Penticton, British Columbia (BC), the Aries Aviation International Piper PA-31 proceeded on an IFR flight plan from Penticton Airport (CYYF), BC, to Calgary/Springbank Airport (CYBW), Alberta, at 15 000 feet msl. When the Navajo was approximately 40 nautical miles southwest of CYBW, ATC began sequencing the aircraft for arrival into Calgary airspace and requested the pilot slow to 150 knots and descend to 13,000 feet. About this same time, the right engine began operating at a lower power setting than the left one. About 90 seconds later, at approximately 13,500 feet the aircraft departed controlled flight and collided with terrain near the summit of Mount Rae killing both people on board. The investigation did not uncover any issues with the pilot’s health, the local weather or the condition of the aircraft that might have led to the accident.
At one point during the crew’s survey work, ATC issued a clearance to 15,000 feet. As the Navajo climbed through 11,000 feet, the pilot switched on the autopilot and switched the fuel selectors from inboard to outboard. Shortly after the aircraft reached 15,000 feet, about 12:56 local time, the pilot selected the fuel pumps on as called for in the aircraft flight manual and left them on for the rest of the flight. The autopilot altitude hold mode was also selected. The pilot then assisted the survey technician in activating the portable oxygen system and, while the pilot put an oxygen mask up to his face, he put it back down a few seconds later.
The autopilot’s altitude hold was disengaged at 13:28 local time. Over the following 47 seconds, the aircraft climbed from 15,000 feet to 15,400 feet before beginning a descent that peaked at 3,200 fpm. Only forward pressure on the control column or a steep pitch selected on the autopilot would have accounted for such a high rate of descent in a Navajo. After nine seconds of descent, the pilot leveled off at 15,000 feet and re-engaged the altitude hold.
At 13:30:55, ATC issued a descent to 14,000 feet. A few seconds later the pilot switched the fuel tanks from the outboard to inboard in accordance with the AFM and selected the autopilot’s pitch mode for the descent. The aircraft entered the clouds during the descent and leveled off at 14,000 feet 90 seconds later, with the pilot re-engaging the altitude hold.
The Effects of Oxygen Deprivation
At 13:33, ATC queried the pilot for his indicated airspeed, but the pilot read back “14,000 feet.” ATC, once again, asked for the speed, but this time the pilot read back the ground speed of 170 knots. The Navajo’s indicated airspeed was actually 140 knots. ATC asked the pilot to slow to 150 knots and descend to 13,000 feet.
A few seconds later, the manifold pressure on both engines decreased, which resulted in the left-hand MP gauge indicating 18 inches of mercury (inHg) and the right engine indicating 15 inHg. After the power reduction, the autopilot hold disengaged while the navigation and pitch mode, as well as the flight director, remained on. The aircraft’s airspeed then decreased to less than the single-engine inoperative best rate-of-climb speed of 97 KIAS. The aircraft remained at 13,900 feet. The stall horn began to sound intermittently when the airspeed reached 91 KIAS. The indicated airspeed then began to increase as the left engine’s manifold pressure increased to 34 inHg while the right engine indicated 20 inHg. The aircraft began a brief descent and the airspeed increased to 100 knots.
Four minutes after ATC asked for the descent to 14,000 feet, the aircraft deviated to the right of the intended track and descended to 13 500 feet before beginning another climb, again slowing the aircraft below safe single-engine inoperative speed. At 1335:43, ATC asked the pilot to confirm that he was headed to the initial waypoint along the route to which he’d been cleared. The stall warning horn was now blaring continuously as the pilot announced a problem with the right engine. Shortly afterward, the aircraft pitched nose-up about 20 degrees and the airspeed decreased to 71 knots. The aircraft then departed controlled flight and entered a right-hand spin.
The Navajo exited the clouds at 13,300 feet while inside the cockpit engine indications showed the pilot had pulled both throttles to idle. As the airspeed increased to 110 KIAS in the spin, the rpm of both engines increased.
Passing through 11,500 feet, ATC asked the pilot to report his situation, to which the pilot declared an emergency. As the aircraft continued spinning, the pilot occasionally added full left and partial right aileron on the control column, control movements that would have actually aggravated the spin. The aircraft completed 7.5 revolutions in the spin before colliding with terrain at 10,000 feet near the peak of Mount Rae. When the aircraft exited the clouds at 13,000 feet, the pilot completed only one of seven spin-recovery steps, reducing the power to idle. As the aircraft continued to descend, the pilot took no further recovery action, except to respond to ATC and inform the controller there was an emergency.
The TSB’s investigation concluded that, during the ascent to and continued flight at 15,000 feet without supplementary oxygen, the pilot likely experienced a headache, dizziness, air hunger, and fatigue, as well as gradual degradations in thinking, memory, judgment, and muscular coordination. His reactions would also have slowed down. His hearing capacity may have decreased while his peripheral vision most likely narrowed as the visual field darkened. The pilot may have found it more and more difficult to perform tasks using working memory or to judge airspeed, heading, and orientation. However, because of the slow and gradual onset of the hypoxic symptoms and the probable associated sense of well-being, it is unlikely the pilot noticed the effects.
A rapid increase in the pilot’s breathing indicated he may have realized something was wrong; however, his response was slow and inappropriate. Specifically, the response to the asymmetric power condition was delayed and the memory-based tasks associated with the single-engine emergency checklist were not performed. As a result of hypoxia-related cognitive and perceptual degradations, the pilot was unable to maintain effective control of the aircraft or to respond appropriately to the asymmetric power condition.
As a result of this accident, Aries Aviation International replaced the oxygen masks in its airplanes with individual cannula-style inhalers. The company ops manual was updated to require all pilots to use supplemental oxygen about 10,000 feet.
