In the 1960s and ’70s most sophisticated aircraft instrument panels were little different from those developed during World War II. Symbology — what there was of it — was white on a black background; attitude indicators comprised a horizon bar and aircraft symbol; directional gyros had to be caged and uncaged periodically because of precession; and pilots had to know how to fly “needle, ball and airspeed” when the vacuum gyros quit.
In spite of the relative crudeness of those old puff, blow and spin instruments, we were all taught — above all — to believe their messages (or some subset of them). You could depend on the instruments and your eyes. Your vestibular system (balance organs) and butt were not to be trusted. There was life in the instruments but unhappy endings in our internal attitude reference systems.

Today’s instruments are colorful and jam-packed with more information than most pilots can digest (See “Less is More”) These systems enable pilots to visualize their environment better then ever, but they can also suffer subtle failures that can lure the unprepared into hazardous situations.
Such was the case on June 18, 2016, at just after 0130 Australian Western Standard Time (WST), when the crew of a Pilatus PC-12 (VH-OWA) almost upset their aircraft in reaction to a faulty synthetic vision system (SVS) display.
The incident was investigated by Australia’s Transportation Safety Board (ATSB) and the story of its findings is rich in lessons for pilots flying aircraft equipped with the newest show-all display systems. What follows is largely from the ATSB investigation into the incident involving the crew of VH-OWA.
The pilot and a check pilot (who was the pilot in command) were setting out on a medical retrieval flight from Meekatharra Airport to Paraburdoo, Western Australia, under an IFR flight plan. Meekatharra Airport is in a remote area, and the airport and its surrounding terrain were dark except for a low-level wash of moonlight. The left seater was the pilot flying (PF); the check pilot was in the right seat; and a flight nurse was seated in the cabin.
Figure 1: This shows the PFD synthetic terrain image with other information overlaid. Pilots in the Pilatus incident said they could not easily separate vital attitude information from the clutter when the terrain image displayed misinformation. Credit: Honeywell

The PF completed the pre-start, start and after-start checks while parked under a metal shed roof; however, the aircraft’s two GPS units had not acquired enough satellites to complete initialization because of the overhead cover. To correct the situation, the pilot taxied the aircraft a short distance onto the taxiway before stopping for the satellite hunt. GPS 1 located all required satellites, but GPS 2 failed to initialize and the crew received an UNABLE FMS-GPS MON caution message. The pilot followed the quick reference handbook actions in response to that message. GPS 2 initialized and the caution cleared. The pilot taxied the aircraft to Runway 9 and initiated a normal takeoff at about 0145.
Some 18 sec. after takeoff, as the Pilatus climbed through 250 ft. AGL at an airspeed of about 110 kt., the pilots observed the radio altimeter (radalt) indication wind down to zero. The radalt low-altitude awareness display on the primary flight displays (PFDs) rose to meet the altitude readout.
Simultaneously, the synthetic vision image on both pilots’ PFDs showed the runway move rapidly left and off the screen, and the ground representation on the PFD appeared to rise rapidly up to meet the zero pitch reference line (ZPRL). The pilot flying reacted automatically pulling back on the control column. The flight path indicator moved up to about 15-deg. No warnings or cautions were sounded or displayed; the stick shaker stall warning did not activate because the aircraft angle of attack (AOA) was not in the shaker range; and the crew received no oral alerts from TAWS.
The pilot flying later told investigators that the synthetic vision image created the impression that the aircraft was sinking rapidly toward the ground, and he responded by instinctively pulling back on the control column. He felt no vestibular sensation that the aircraft was descending, nor had there been any indication of a strong wind that could have caused the aircraft to drift off the runway centerline. The resulting sensory confusion caused the PF to experience a level of motion sickness.
The check pilot immediately looked outside — there was no standby instrument on the right side of the cockpit — and was able to discern a visible horizon due to the moonlight. He cautioned the PF that the aircraft had a nose-high attitude, which prompted both pilots to switch their focus to the electronic standby instrumentation system (ESIS) and closely monitor the attitude and the airspeed tape. The PF lowered the aircraft nose to regain an 8-deg. pitch attitude. Airspeed had fallen to 101 kt. during the incident and increased back to the target airspeed of 110 kt. as the pilots adjusted the pitch attitude.
The aircraft continued to climb and as it passed 850 ft., the synthetic vision display corrected itself and all indications returned to normal. After retracting the landing gear and flaps, the PF deselected the synthetic vision mode on the left PFD. The check pilot continued to monitor the synthetic vision on the right-side PFD, and the issue did not recur during the flight. The aircraft subsequently landed at Paraburdoo Airport without further incident.
The Investigation
Investigators immediately turned their attention to the radio altimeters and SVS. The first indication of trouble spotted by the pilots was the unwinding of the radalt display and the rising radalt caution crosshatches on the PFD. (See Figure 1.)
An engineering assessment determined that both radio altimeter antennas — one for transmit and one for receive — had been in service for over 9,000 hr., and had failed. The antennas did not have life limits but were required to be replaced “on condition,” which essentially meant that the antennas remained in service until they failed.
When the altitude displayed on the radalt is below 550 ft. AGL, low-altitude awareness is displayed using diagonal yellow lines (Figure 1). During this incident, the crew noticed that the low-altitude awareness symbology was displayed.
The radalt display is shown in green numbers on the PFD when the radalt data is valid and less than 2,500 ft. If the radalt data becomes invalid, the radalt digital readout is replaced with a radar altitude data (RAD) annunciator and an amber RA 1 FAIL crew alerting system (CAS) message is displayed. The crew received no annunciations during this incident to indicate that the radalt had failed.
The ATSB believes failure of the radalt antennas likely resulted in the radalt winding down to zero, and the radalt low-altitude diagonal bars to appear on the PFD altitude tape to show the aircraft was close to the ground.
Perhaps more important, the radalt information was used in conjunction with the runway and obstacle information in the TAWS database to feed the synthetic vision system. This resulted, says the ATSB, in the runway appearing to rise up toward the aircraft reference symbol on the PFD.
The synthetic vision system display is depicted in Figure 2. The PFD image provides a three-dimensional representation of surrounding terrain, obstacles and runways based on a terrain database. Normal attitude, altitude and airspeed information is overlaid on top of the terrain display. The TAWS terrain database provides geometric altitude (obtained from the GPS) in order to display synthetic vision terrain and terrain-related items such as runways and obstacles. During this incident, the synthetic vision system provided no failure annunciations.
Figure 2: This is a close-up of the low-altitude awareness radalt crosshatch display that rises from bottom of the display as radar height above the surface decreases from 550 ft. to zero.

The aircraft and avionics manufacturers warn pilots that the synthetic vision system is not to be used for primary input or navigation. (See Figure 3.) A similar warning was contained in the Honeywell Primus Apex Smart View supplement to the aircraft flight manual.
Figure 3: Pilots are warned in aircraft publications that synthetic vision images are not to be used for terrain or obstacle avoidance. Some Pilatus pilots tell BCA that they disable synthetic vision for takeoff and departure.

Both pilots told investigators that they were aware that the synthetic vision system should not be used for primary navigation. Interestingly, the SVS is automatically activated at start-up but can be deselected by the pilot.
Aircraft Reference Symbols
The pilot was using the flight path indicator on the SVS. This consists of the flight director command bars (magenta symbol in Figure 4) and the flight-path aircraft reference symbol (green symbol in Figure 4). The flight path indicator is a path-based mode and depicts the aircraft’s predicted flight path (not just aircraft pitch) and is affected by pitch attitude and the aircraft’s ground speed. It shows flight path angles — up for increasing and down for decreasing flight path angles, whereas the traditional pitch-based mode depicts aircraft pitch angle. The flight path angle depicted in Figure 4 is 4-deg. nose down.
Figure 4: This image shows a flight path angle of -4 deg. Notice the flight director command bars and the zero pitch reference line.

During the departure incident, the movement of the runway to the left of the screen was probably associated with a small displacement of the aircraft to the right of the runway centerline. As the radalt senses that the aircraft is nearing the ground, smaller lateral deviations from the runway centerline generate significant movement of the synthetic vision runway image.
Engineers replaced both radalt antennas and also the radar transmitter/receiver on the incident aircraft. No subsequent similar event has occurred. The engineers also replaced the GPS 2 antenna due to slower than normal acquisition of satellite navigation after power up, and updated the GPS databases, although it was considered that these did not contribute to the incident.
The Pilots
The two pilots were highly experienced; the left-seat pilot had over 11,000 hr. total aeronautical experience and over 2,600 hr. on the aircraft type, and the check pilot had over 15,000 hr. total experience and 3,000 hr. on type.
Both pilots told investigators that they had previously experienced failure of primary flight instruments at low level and at night in different aircraft (without synthetic vision systems). They had been able to disregard the erroneous or failed instruments and reference the standby instruments to maintain control of the aircraft and situational awareness. However, the pilots told the ATSB that the prominence of the synthetic vision display was so prominent that it was difficult to ignore the erroneous information and locate valid information. Additionally, the pilot flying reported feeling a level of motion sickness, probably associated with the combined effects of the prominent synthetic vision display and conflicting vestibular sensory information.
The combination of the runway and the radalt tape moving up gave the very strong illusion that the aircraft was going to hit the ground. The PF reported that he realized something was wrong but could not initially identify it. The image of the ground rising up and the runway disappearing rapidly sideways took his focus away from anything else.
The PF commented that the check pilot’s caution “attitude” helped to redirect his attention to the standby indicator. The check pilot could not easily see the standby indicator. Both pilots commented that the situation may have been more serious if operating single-pilot or if they had already flown more sectors that night and been more fatigued.
The pilots stated that it was impossible to discern the valid attitude information on the PFD (overlaid on top of the synthetic vision) and revert to flying “power and attitude” given the prominence of the erroneous synthetic vision information. While it is possible to deselect the synthetic vision, it requires two button presses or the use of the cursor control device to do so. That is very difficult to do at low level while maintaining control of the aircraft — keeping the right hand on the thrust lever and the left hand on the control column.
Honeywell issued a Pilot Advisory Letter in response to this incident reminding pilots to look at the primary flight indications presented on the PFD at all times. The incident pilot commented that the letter should have referred pilots to the standby attitude indicator instead. The PFD display at the time of failure was simply “too confusing to start looking for two small, white attitude bars.” Similarly, to break the fixation on the erroneous information, it is important to look somewhere else at a different instrument — the standby indicator, said the pilot.
The ATSB reminds pilots that spatial disorientation can occur when visual cues provide sensory inputs that are not matched by the motion sensed by the pilot through the vestibular senses. In this case, said the Safety Board, discrepancy between the visual display showing the aircraft apparently descending toward the ground, and the lack of any consistent physical sensation, led to disorientation. The flight was conducted at night, and the pilot at the controls did not look outside for a visual reference. The check pilot did look outside and found that there was enough moonlight to provide some visual reference sufficient to show the aircraft pitch and roll attitudes relative to the horizon.
Spatial Disorientation
The ATSB research report “An Overview of Spatial Disorientation as a Factor in Aviation Accidents and Incidents” describes the spatial disorientation suffered by the crew in this incident. That is, the pilot identified that they were sensing erroneous information. The conflict between the pilots’ own perceptions and that given by the instruments alerted them to a problem, which they were then able to address. However, the crew reported feeling some level of disorientation stress, or motion sickness, which is indicative of a disagreement between the senses.
The visual system provides about 80% of orientation information, hence the overriding presence of incorrect visual information deprived the pilots of the majority of orientation information.
Other factors such as tiredness or fatigue, and high workload, can contribute to a pilot’s ability to assess and effectively deal with spatial disorientation. Both pilots commented that they wanted to share their experience because if they had been operating single pilot or near the end of a long shift, recovery from the instrumentation failure may have been much more difficult.
In addition, if the outside light conditions had been completely dark due to a lack of any moonlight in an area without terrain lighting, or the aircraft was in cloud, recognition of the spatial disorientation would have been reliant on the pilots being able to either extract the basic attitude, altitude and airspeed information from the primary display ignoring the background image, or revert to the accurate information depicted on the smaller standby indicator.
Pilots operating under Instrument Flight Rules are trained to focus their attention on the visual information presented by the aircraft instruments and to “believe” that information rather than the sensory information from the vestibular system, which can provide misleading cues.
The ATSB research report stated “instrumentation should present a clear and intuitive sense of position, which the pilot under conditions of high stress and workload can instantly achieve an idea of what the aircraft is doing.
“Failure of the aircraft instruments should hopefully never occur. However, in the event that it does, the pilot needs to receive clear and non-ambiguous indications of instrument failure. If a key instrument fails, such as the attitude indicator, the pilot needs to know that it has failed so they no longer depend on its information.”
As part of the safety actions Honeywell is investigating ways to make its synthetic vision system more robust against a similar failure. The focus of the company’s investigation is to prevent the synthetic vision display “from continuing to display the image when the data is incorrect but assessed as valid by the radar altimeter.”
Meanwhile, the ATSB reminds pilots that incorrect instrument indications that are not associated with a failure mode present pilots with a complex and challenging situation. This situation may be exacerbated during single-pilot operations, where there is a lack of external visual references (such as at night or in IMC), under high pilot workload conditions, or where a pilot is experiencing an elevated level of fatigue.
“The image of terrain on the PFD is powerful and compelling,” says the ATSB. “This incident highlights the manner in which an inaccurate synthetic vision image can rapidly lead to a degree of spatial disorientation. Pilots need to ensure that they are familiar with the limitations of the SVS and how to effectively deal with erroneous information as well as system failure modes. Organizations that operate aircraft fitted with similar technology should ensure that appropriate information and training is available to pilots, including when and how it should be used when it is not approved for primary navigation.”