Pitch-up Illusion on Take-Off

26 Mar 21 9 Comments

On the 28th of April 2017, a single-engine turbo-prop flying at night under instruments crashed near Amarillo International Airport in Texas. The accident report had been available on the app.ntsb.gov site but appears to have been removed. You can download a copy using the CAROL query tool for the NTSB number CEN17FA168. This piece was written using my copy of the final report and the Report Docket.

Google Earth view of radar data and simulation trajectory

The aircraft was a Pilatus PC-12, a single engine aircraft introduced in 1994 with a large payload and high cruise speed across long distances. The Pilatus PC-12 can be used in “rugged environment” with rough landing strips. The PC-12 is popular for corporate transport, as the “comfortable” configuration can take nine passengers. Flying magazine described it as “more economical to operate than any turbine airplane of similar size.”

This specific Pilatus PC-12, registration N933DC, was operated by Rico Aviation Ltd in the US under Part 135 (commercial, non-scheduled aircraft operations). The pilot was based at Amarillo International.

On that evening, the PC-12 was operating as an air ambulance flight, with the pilot and two medical crew members on board. The flight was scheduled to Clovis Municipal Airport in New Mexico to pick up a patient. The patient would then be transported to a medical facility in Lubbock, Texas.

The pilot did his flight preparation while waiting for the medical facility to be ready to receive the patient. He had almost 6,000 hours flight time, although just 73 on this make and model. He started his aviation career as a mechanic, progressing to repairman and flight engineer before qualifying as an Airline Transport Pilot license and Flight Instructor. His management and co-workers considered him to be a trustworthy pilot; the only negative point was a report from FlightSafety which described him as “behind the plane” during simulator training. Both the Chief Pilot and the Director of Operations had flown with the pilot recently and confirmed that they had no concerns or issues with the pilot’s flying skills.

His weight and balance submission for the flight from Amarillo included a 180-pound patient and 1,800 pounds of fuel, although he had no patient on board (yet) and normally should have departed with 2,000 pounds of fuel. The Chief Pilot of the operator believed that the pilot may have entered the weights that he was expecting to be departing from Clovis with.

At 23:32 local time, the pilot contacted Amarillo ground control to confirm he had ATIS (automatic terminal information service) information Oscar and requested IFR clearance to Clovis, New Mexico.

The ground controller issued a clearance “as filed”, meaning that the clearance was for the routing as submitted by the pilot for his flight plan, confirming that he had an initial climb to 8,000 feet above mean sea level and a transponder code of 4261. The pilot read back the clearance.

A few minutes later, the Rico Aviation medical dispatcher confirmed final acceptance of the mission. The pilot contacted Amarillo ground control again and requested taxi to runway 04 for departure. The flight was cleared for take off at 23:43.

As the aircraft climbed away from Amarillo, it started a right turn towards Clovis. The local air traffic controller noticed that the aircraft was broadcasting the wrong transponder code. He called the pilot to let him know and repeated the correct code: 4261

The aircraft levelled off at between 800 and 1,000 feet above ground level for about thirty seconds and the correct transponder code appeared on the controller’s radar. The controller confirmed this with the pilot and the PC-12 resumed its climb. The controller saw the aircraft disappear into the clouds and asked the pilot to contact departure.

The pilot changed frequencies to departure control and reported in as “with you at 6,000”. For his location, six thousand feet above mean sea level was about 2,400 feet above the ground. There was no sound of distress or mention of an issue.

A few seconds later, the PC-12 entered a rapid descent. Before any one had time to react, it disappeared from radar. The departure controller immediately attempted to contact the pilot but received no response.

About 1.5 miles south of the airport, the PC-12 crashed into a pasture and then burst into flames. There were no survivors.

Radar data showed that the aircraft had reached a speed of 17,000 feet per minute (descending at over 300km/hour or almost 200 mph) before crashing. A surveillance camera at a nearby truck stop recorded the lights of the aircraft descending at an angle of about 45° followed by an explosion.

The wreckage did not show any evidence of problems with the flight control system. The damage to the propeller hub and the bent blades made it clear that the engine was operating under high power when the aircraft impacted the filed. The ground scars and the damage to the aircraft, confirmed by the video footage, showed that the PC-12 was in a steep descent, nose low and wings level.

As far as anyone could see, the pilot literally flew a fully functioning aircraft into the ground. While the aircraft was climbing to 6,000 feet, the peak pitch angle was about 23° nose-up. But about 37 seconds before impact, the pitch angle decreased steadily until it reached 42° nose-down.

The pilot was the victim of a relatively rare but often fatal spatial disorientation known as the “pitch-up illusion” or more correctly, somatogravic illusion.

We have an inbuilt spatial orientation system which allows us to perceive motion and our three-dimensional position in relation to our environment. Our sense of orientation allows us to instinctively know where our limbs are and which way is up.

Generally speaking, we rely on visual cues to determine our orientation: we know where we are based on what we see around us. The visual system is reinforced by additional information supplied by the vestibular system, the balance organs located in our inner ears, and the proprioceptive system, also known as the “seat of the pants”, where tendons, muscles and joints detect control forces and pressure. The information collected by these three systems is integrated into a single model of orientation and is usually amazingly accurate.

However, our internal systems were designed for walking on the ground, not flying through the air in a Pilates PC-12.

Visual information supplies some 80% of the data used to define our orientation, with the remaining 20% split between the vestibular system and the proprioceptive system. A pilot who cannot see the horizon or other points of reference has a wildly less accurate understanding of his or her orientation. The vestibular system and the proprioceptive system are much less reliable and can lead us to experience a number of spatial illusions. Worse, because we are used to our sense of orientation happening subconsciously and very reliably, we are instinctively reluctant to consider outside information which might help to correct the illusion.

One of the side-effects of the way the vestibular system works is the somatogravic illusion. The vestibular system consists of the semi-circular canals and the otolith organs. The otolith organs are actually a clever biological linear accelerometer: small grains of calcium carbonate embedded in a gelatinous substances move forward and backward which tickles the end of small hairs, sending a signal to the central nervous system.

If you tilt your head up, the grains shift backwards. If you accelerate, the grains also shift backwards, bending the small hairs in a very similar way. Because our bodies have not evolved for fast acceleration, the central nervous system can misinterpret this data to mean that your position is pitching up as opposed to accelerating. If there is no other useful data from the other systems, there is nothing to correct this.

When an aircraft is taking off, you are accelerating and pitching up. In the dark with no visual references, the central nervous system is prone to interpreting sustained acceleration as tilting, so that you feel that you (or in this case the aircraft) are pitching up more than you actually are. In one example that I read, an aircraft accelerating from 170 to 200 knots over a period of ten seconds feels the same as pitching 9° nose-up.

So taking off in the dark, the central nervous system is quite clear that the aircraft is pitching up more excessively than it actually is. In response, the pilot presses forward on the control column to return to the normal climb. At the point where the pitch feels correct to the pilot, the aircraft is actually pitching down.

Researchers have explored somatogravic illusion since the 1940s, after a startling number of pilots taking off at night ended up pitching the aircraft nose-down and crashing into terrain. In modern times, it is most commonly associated with go-arounds at night, a high stress situation where it is easy to get distracted. Because it takes place while climbing away in non-visial conditions, it is often fatal before the pilot has a chance to consider the instruments and realise that he or she has fallen prey to an illusion.

On that night in April 2017, the pilot seemed to be lightly distracted or perhaps fatigued: broadcasting the wrong squawk and then levelling briefly as he updated his transponder. This would make him that much more susceptible to spatial illusions. He changed frequencies to say “with you at 6,000” although he had been cleared to 8,000 feet, and should have reported climbing through 6,000 feet. It seems likely that he was levelling out at this point in the climb.

But a bigger clue turned up in the maintenance logs, which showed that there had been recurring issues with the autopilot, a Honeywell KFC 325. There was a specific fault which affected a number of PC-12, in which the autopilot lost the ability to adjust the horizontal stabiliser trim. In the cockpit, the master warning light would illuminate accompanied by the continuous beeping of the autopilot-trim warning tone. In addition, a red TRIM light on the autopilot mode control panel and a red A/P TRIM light on the Crew Alerting and Warning panel would illuminate. The autopilot would automatically disengage.

This fault had been logged twice in the previous five months, once by the accident pilot and once by another pilot. The other pilot said that in order the stop the continuous beeping and extinguish the warning lots, he would press the autopilot test button on the autopilot mode control panel.

The Chief Pilot confirmed that there was a continuing issue with the autopilot and mentioned the same fix.

It would often disconnect unexpectedly, triggering a master warning tone. It would require the pilot to reset the system by pushing the autopilot test button, then re-engaging the autopilot.

Two days before the accident, the Chief Pilot had recorded a video of the autopilot disconnecting during a flight. Analysis of the video after the event concluded that the autopilot’s pitch trim adapter likely lost power for a moment, which caused the autopilot to disconnect. In the video, the Chief Pilot recovered by using the test button.

Excerpt from Rico Aviation PC-12 Emergency Procedures

According to the manual, in this situation, the the pilot should disengage the autopilot and then remove power by pulling out the autopilot circuit breaker. There was no reference to using the TEST button or reinstating the autopilot.

On the night of the crash, the aircraft was in a climbing right turn when ATC contacted the pilot about the transponder code. The aircraft levelled off briefly while the pilot updated the code, thus the autopilot was not likely to have been engaged at that moment.

As the aircraft climbed through 1,000 feet, the pilot almost certainly engaged the autopilot, as was standard procedure at the airline.

The wind was from the north, 21 knots gusting 28, and moderate turbulance was reported in the area. The turbulance might have stopped the pilot from noticing that the autopilot had disconnected. It certainly would have contributed towards a feeling of spatial disorientation.

The aircraft continued to climb, reaching a peak pitch angle of about 23° nose-up at 37 seconds before impact.

A few seconds later, the pilot reported to ATC that he’d reached 6,000 feet.

At 31 seconds before impact, just after the report to ATC, the aircraft pitched down. At the same time, the roll angle increased to the left. It is clear that the autopilot was not connected, or else it would have correct the roll.

It seems clear that the pilot was pitching down based on the feedback from his spatial orientation systems which, in the dark, was misinterpreting the continued acceleration as a pitch up event. The aircraft’s pitch decreased steadily to 42° nose-down with the roll reaching 78° to the left. The pilot had just seconds to realise what was happening and to correct it.

Finally, the light bulb filament showed that the “autopilot disengage” caution indicator was lit at the point of impact.

The amber A/P DISENG caution message will illuminate 3 seconds after the signal input to the CAWS changes from 28V (A/P engaged) to 0V (A/P disengaged) and the Control Wheel Steer (CWS) button is not pressed. The caption will remain illuminated for about 26 to 27 seconds; it extinguishes at a maximum of 30 seconds from the initial time of the autopilot disconnect.

The autopilot disengage caution indicator remains illuminated for thirty seconds after the autopilot disengages, thus we know that it must have been disengaged within 30 seconds of impact. We also know that it was not engaged at the point when the aircraft started its descent and roll to the left.

Thus, at some point after the pilot pitched nose-down, he attempted to engage the autopilot. This was almost certainly an attempt to use the instruments and to correct the unusual attitude that the pilot realised that the aircraft was in.

What we’ll never know is whether the pitch trim adapter lost power in the last 30 seconds of the flight, which would have disengaged the autopilot, or whether the autopilot was engaged too late to make a difference. At nine seconds before impact, the aircraft’s limit load factor was exceeded which would automatically disengage the autopilot. If the pilot had only turned it on a few seconds earlier, there would not have been time for the autopilot to correct the flight attitude.

Wreckage organised in hangar

From the report:

The apparent pitch and roll angles, which represent the attitude a pilot would “feel” the airplane to be in based on his vestibular and kinesthetic perception of the components of the load factor vector in his own body coordinate system, were calculated. The apparent pitch angle ranged from 0° to 15° as the real pitch angle steadily decreased to -42°, and the apparent roll angle ranged from 0° to -4° as the real roll angle increased to -78°. This suggests that even when the airplane was in a steeply banked descent, conditions were present that could have produced a somatogravic illusion of level flight and resulted in spatial disorientation of the pilot.

Probable Cause

The pilot’s loss of airplane control due to spatial disorientation during the initial climb after takeoff in night instrument meteorological conditions and moderate turbulence.

You can download a copy of the final report by searching the CAROL query tool for NTSB number CEN17FA168.

It is easy for us to say that a pilot must always rely on instruments and not rely on his or her own perception. This dismisses the point that we have complex systems which define our sense of time and place; it isn’t something that we consider or conclude. Because we instinctively know where we are in relation to the earth and each other, we resist taking in new information which proves that our sense of our position or motion is wrong, a particular risk in three-dimensional space.

Studies have shown that pilots with more experience are just as susceptible to spatial disorientation and that somatogravic illusion is most often suffered by pilots who hold instrument ratings. This seems obvious, in that instrument rated pilots would be more likely to fly at night and, more importantly, would more likely have survived to tell the tale.

One study concluded that pilots fall into two categories: those who have suffered from spatial disorientation and those that haven’t …yet.

Category: Accident Reports,

9 Comments

  • “the only negative point was a report from FlightSafety which described him as “behind the plane” during simulator training.”
    – what does that mean? I’m not familiar with that phrase.

    “This suggests that even when the airplane was in a steeply banked descent, conditions were present that could have produced a somatogravic illusion of level flight and resulted in spatial disorientation of the pilot.”
    – what would engine rpms have done at that time, and wouldn’t the pilot hear the rpms increasing dramatically?

    • A pilot who is “behind the plane” is reacting to things happening rather than anticipating what is going to be needed. I had this problem a lot when I would take a break from flying the Saratoga: I would be struggling to keep up with what I needed to do because I never had time to think about what was going to happen next. Basically, it means you have a much higher workload and could struggle to react to an unexpected situation.

      The engine rpms would have increased but at the time when the illusion takes hold, you are flying with full TO/GA power so expecting high RPMS. With a noise-cancelling headset, I suspect the chances of you hearing the increasing engine noise over a thirty second period are pretty slim; especially taking into account that a hyperfocused pilot trying to fix a problem will often ignore much more blatant sounds like blaring alarms.

  • I’ve seen a lot of photographs of aircraft wreckage reassembled for accident investigations. But I’ve never seen one without a single piece that was recognizable as part of an aircraft. That reconstruction is just a heap of scrap metal. Chilling.

  • It’s always chilling to read about accidents caused by somatogravitic illusions – it’s like your own senses betraying you, in a way. That’s one reason why pilots need instrument training to fly in the dark (close to midnight in this case)!

    I had to look up what a Pilatus PC-12 looks like when it’s intact; quite a smart little plane!

    The report is at https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/95076/pdf . It looks like the NTSB is reorganizing their public database every few years?

  • “Behind the plane” may be a misnomer for this sad accident. Sylvia explains it very well, but I don’t think that it was a main cause in this particular case. In the event of an accident, the training records of a (commercial) pilot will be scrutinised by investigators and anything negative would come to the fore. Of course, I may be wrong.

    There is a bit of a joke abut it, sorry I don’t intend to poke fun at anyone who died in a crash, it is an old Learjet joke. The 20-series was known as a “hot ship”. Under normal operating circumstances a climb rate of more than 8000 feet per minute was nothing unusual, and even with an engine failure at V1 the aircraft would still be able to clear obstacles with ease. Due to the modest sweep of the leading wing edge of 23 degrees, its MMO was limited to M .81, and the crew had to learn to cope with a lateral oscillation on finals. Yes, a very satisfying aircraft to fly but it required some proper training and understanding of the characteristics that the design imposed. Many private owners just got a check-out on the aircraft and, if they passed the check-ride, would fly it, often single pilot. It would have been later when sophisticated simulators would become available
    .
    The original Learjet had straight jets, requiring (very) high altitude operations. I have on one occasion burned 500 pounds (!!) of fuel during taxi to the holding point. We climbed as quickly as possible (which was very quickly) to a high cruising altitude, or our range would be severely curtailed. When the Learjet entered service, few pilots outside the military had experience with high altitude flying, and the Learjet could even outperform fighter planes of its day. (Too) many PPL owners cruised at FL 410 or above without any understanding of the consequences of exceeding the MMO, nor having ever had any training about flying in the “coffin corner”. The Lear was certified up to 51000 ft (FL 510).
    Not surprisingly, the Lear got a bit of a bad press as an accident-prone design

    The joke goes like this: Learjet captain to F/O: “I am not satisfied with your performance, you are always ten miles behind the aircraft.”
    F/O: “That is fine with me captain, I will watch you crash it from ten miles behind.”

    The sad accident in this article has some eerie similarities with a crash that took place in November 1984, involving a Rockwell Turbo Commander, EI-BGL. It was operated by a Dublin-based company, Flight Line Ltd. After the crash this company ended its operations and the name has since been adopted by another, not related company.
    I was at the time flying Corvette EI-BNY which was managed by Flight Line on behalf of Guinness Peat Aviation. GPA was founded by the late Dr. Tony Ryan who later founded Ryanair. So I knew the pilot, Jack Walsh. Jack was a competent commercial pilot and I doubt that he would have been sent on a single-pilot flight at night if the Ops. Manager, the late Tony Doyle, had any reason to doubt his competency. Meaning: I do not think that being “behind the aircraft” was in Jack’s training records.

    Both accidents, the Pilatus and the Turbo Commander, took place at night. Both were operating in conditions without outside light and few, if any, visual references to the real horizon. In both cases there were no obvious mechanical defects that would have caused the crash, but in both cases there is the strong suspicion that the pilots were distracted by a sudden, un-commanded disconnection of the autopilot. A contributory factor, in both cases, was the suspected spatial disorientation (“vertigo”) that affected the pilots when moving their head to search for the cause of the abrupt and unexpected systems failure. Both aircraft entered a steep dive and flew into the terrain at high speed.
    Neither accident was survivable.
    The Commander broke up in mid-air.

    I remember clearly having suffered vertigo myself.
    We were flying into Frankfurt am Main (EDDF), the runway was probably 25R (in those days 25L still was “owned” by the USAF).
    It was autumn, late afternoon. The sun was low and there was some haze. The visibility was probably 5 miles, but looking into the sun it was a lot less. I had visual contact and was hand-flying, still on the ILS but relying on outside visual cues.
    There was a motorway (maybe still is), bisecting my flight patch at an angle of about 45 degrees. The real horizon was not visible. When the motorway came in sight, it became my surrogate horizon and I suddenly suffered vertigo, badly. I had enough experience to realise what was happening and I instantly reverted back to the instruments inside. I recovered immediately and could continue my approach normally.
    It was a very good and actually very useful lesson.

    The somatogravic illusion is widely used, its effects programmed into the motion systems of flight simulators where e.g. a pitch movement, accompanied by steady visual cues from the outside world, projected on the windscreen, and the inside by the flight director, can create the illusion of, e.g. acceleration or deceleration. Even if the motion is stopped, the ear is still trying to make sense of the motion signals and the brain is tricked into processing them all in a way that makes a simulator present not only visual, but also “seat-of-the-pants” cues that the trainee experiences as being close to reality.
    So close, in fact, that training in the latest generation of simulators is so realistic that it can obviate the need to fly the actual aircraft.

  • Totally unrelated to this article, it is about covid-19.
    Just to break the seriousness of this posting.

    The captain of an airliner welcomes his passengers:
    “Ladies and gentlemen, this is your captain. Welcome on board.
    My name is Joe Bloggs and I am working from home today.”

  • Similarities with AF447?
    In the sense that an aircraft that had no major defects flew into terrain, well I can buy that.
    But there are differences, substantial ones.
    The Pilatus as well as the Commander were operated single crew at night.
    In both cases, the probable cause was a not selected, accidental disengagement of the autopilot. The sole pilot was taken by surprise and while trying to figure out what was happening suffered spatial disorientation, vertigo. It is likely that in both cases the pilots tried to regain control, and overcorrected. The airspeed was already building up during the attempted recovery, The aircraft suffered structural damage – the Commander’s RH wing and tailplane even detached from the airframe in mid-air, the starboard propeller had sliced through the cabin and the port wing also was deformed. All well before impact.
    The A340 is a large airliner. If I remember correctly, there was a mismatch between some instruments causing the autopilot to disengage. The captain was in the crew sleeping bunk, at the controls were the F/O and a junior second officer.
    There was a total lack of coordination between the two pilots who at the time were at the controls. It seems that there was no structured effort to figure out what was happening. Even with the loss of ASI, any reasonably competent pilot should have been able to keep the pitch attitude stable and with the correct power setting, even if the aircraft had not maintained the correct flight level, should have had enough time to call the senior cabin attendant to get the captain back on the flight deck.
    Instead, the two pilots were fighting one another’s actions. The F/O made correct inputs, but the 2nd officer counteracted them. Since the Airbus has fly-by-wire control systems, it is possible for one joystick to be moved in the opposite direction to the other. In one particular aircraft that I have flown, the action would and should have been what we called “the captain is king”, meaning that control can be selected to eliminate and override inputs from the other pilot’s station.
    I am not familiar with the various “laws” that have been woven, or programmed, into the Airbus control systems but in any case it is not very likely that the pilots, even the junior second officer, would not have been given some basic training on the type.
    AF447’s descent path was erratic, between stalling and high speed. By the time the captain had woken up and found his way back into the cockpit it was too late.
    So in my opinion, there was the superficial commonality of an aircraft flying into terrain (the sea) at night, but there were many factors that made it a different scenario.

  • I agree Rudy, many differences but surely the fundamental cause in both cases was the spatial disorientation of the PF causing him to fly a serviceable aircraft into terrain.

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