The autopilot had been engaged in the vertical speed mode for the climb, and the pilot was using a portable electronic device (PED) as the Cessna Citation CJ2+ neared the assigned cruise altitude. As a result, he did not notice that the aircraft’s airspeed had decreased and that its pitch attitude had increased significantly. The stall warning system did not provide a warning before the autopilot automatically disengaged and the Citation departed from controlled flight over central England the morning of Dec. 31, 2013.
After stalling, the aircraft rolled several times before the pilot was able to regain control, according to the accident report by the U.K. Air Accidents Investigation Branch (AAIB). Neither the pilot nor his passenger was hurt, but the Citation’s wings were substantially damaged by overload forces during the upset.
The pilot, who also owned the U.S.-registered light jet, had 3,900 flight hours, including 600 hours in type, and held both U.K. and U.S. private pilot certificates. “He completed initial type rating training for the Citation CJ series at a major training provider in Wichita (Kansas, U.S.) in 2006 and had returned to this provider for annual recurrent simulator and ground school training,” the report said.
“In addition, he had completed a ‘jet upset’ course in an L-39 [Aero Vochody Albatros] aircraft in Albuquerque, New Mexico, in 2006. This comprised ground school and two flight sorties of 0.6 hours each,” the report said.
‘Flash of Frost’
The Citation had been kept in a heated hangar at Leeds Bradford Airport the night before the planned flight to Palma de Majorca, Spain. There were scattered clouds between 600 ft and 1,000 ft, and broken ceilings between 2,000 ft and 8,000 ft over the airport, and the freezing level was about 4,300 ft. The report did not provide information on visibility or ceiling.
The pilot found nothing amiss during his preflight inspection of the aircraft. He then took the left cockpit seat, and his passenger occupied a seat on the right side of the cabin and donned a three-point harness. “Three small dogs were in the cabin, unrestrained, on and around the passenger’s lap,” the report said.
The Citation was within weight-and-balance limits when it departed from Leeds. “The pitot and static heat were selected ‘ON’ before departure, and the takeoff and initial climb were without incident,” the report said. “The aircraft followed a southerly track, climbing continuously towards the planned cruising level of FL 430 [Flight Level 430, approximately 43,000 ft],” the report said.
The pilot later told investigators that one brief encounter with icing conditions occurred during the climb. He described it as a “flash of frost” across an unheated area of the windshield.
“The engine anti-ice had been selected ‘ON’ earlier in the climb, and the pilot then selected the wing and tail de-ice as a precaution, although no ice was seen on the wing at any time,” the report said. “The engine, wing and tail ice protection systems were selected ‘OFF’ later in the climb.”
Visual flight conditions prevailed above the lower cloud layers, but the Citation encountered cirrus clouds as it neared the cruise altitude.
Vertical Speed Mode
Cessna Citation CJ2+
Cessna Aircraft began development of the Model 525 CitationJet in the 1980s as a replacement for the original Model 500 Citation introduced in the early 1970s. Major differences include a laminar-flow wing, a T-tail and Williams International FJ44 engines. Like its predecessor, the CitationJet and its successors can be certified for single-pilot operation.
The CitationJet entered the market in 1993 and later was renamed the CJ1. Avionics upgrades, larger cabins and improved performance have marked the introduction of the CJ1 and successive models: the 525A CJ2 in 2000, the 525B CJ3 in 2004 and the 525C CJ4 in 2010. A plus sign (e.g., CJ2+) designates models equipped with full authority digital engine control (FADEC).
Specifications for the CJ2 include maximum weights of 12,500 lb (5,700 kg) for takeoff and 11,525 lb (5,228 kg) for landing; maximum climb rates of 4,120 fpm with both engines operating and 1,004 fpm with one engine inoperative; a ceiling of 45,000 ft; maximum cruise speeds of 418 kt and 0.737 Mach; range of 1,613 nm (2,987 km); and a stall speed in landing configuration of 86 kt.
Sources: Cessna Aircraft Co., Wikipedia
The pilot conducted the climb with the autopilot engaged in the vertical speed mode and initially with a selected climb rate of 2,000 fpm. The Citation was equipped with full authority digital engine control, and the pilot had placed the thrust levers in the maximum continuous thrust detent.
Simply stated, while climbing in the vertical speed mode, the autopilot will progressively increase angle-of-attack to maintain the selected climb rate, typically resulting in a decrease in airspeed.
“The autopilot will, in this mode, prioritise maintaining vertical speed over airspeed, and pilot vigilance and intervention is required to avoid a low-speed condition,” the report said. “As the aircraft was operating at the edge of its climb performance envelope, there was insufficient thrust to follow the selected climb profile.”
The report said that the pilot had selected the vertical speed mode for the climb because he previously had noticed that the autopilot tended to “hunt” (i.e., make continuous small changes in pitch attitude) with the flight level change mode engaged. [In this mode, the autopilot adjusts the aircraft’s pitch attitude to maintain the airspeed selected by the pilot.]
“Whilst the use of VS [vertical speed] mode in the climb is not prohibited by the AFM [aircraft flight manual], it exposed the aircraft to the risk of entering a low-energy state during the climb,” the report said. “Without greater systems knowledge, the pilot was unaware of the additional risks involved in the use of VS mode. Therefore, he was unable to make an informed decision regarding this autoflight mode.”
The aircraft did not have, and was not required to have, a cockpit voice recorder or a flight data recorder. However, it was equipped during manufacture in 2010 with an aircraft recording system (AReS). “This was fitted as a system diagnostic and troubleshooting tool but in this instance was also available for accident investigation purposes,” the report said.
The aircraft’s Enhanced Ground Proximity Warning System (EGPWS) also provided data for the investigation.
The recorded data showed that the pilot selected lower climb rates, in 500 fpm increments, as the climb progressed. As the aircraft passed through FL 410 at 1057 local time, the selected climb rate was 1,000 fpm and indicated airspeed had decreased below 150 kt.
“The indicated airspeed continued to decrease, reducing below 140 kt 46 seconds later,” the report said. “The pilot noted that the indicated airspeed was lower than he had expected, with the green ‘donut’ marker on the speed tape [which indicates an airspeed value equal to 1.3 times stall speed] being slightly faster than his actual airspeed. He therefore reduced the rate of climb to 500 fpm.” Indicated airspeed at this time (1059, about one minute before the upset) was 128 kt.
“Based on his experience, the pilot considered that the selection of a vertical speed of 500 fpm should have managed the aircraft’s energy sufficiently to achieve FL 430 without incident,” the report said. “However, recorded data shows that over the next 50 seconds, the speed gradually reduced by a further 10 kt.”
‘Violent and Confusing’
The Citation was nearing FL 430 when the pilot decided to check the wind information displayed on his primary flight display (PFD) against the forecast wind information that he had saved on his PED, a tablet-style computer.
“He therefore looked down to the PED, located on the unoccupied right cockpit seat, for what he believed to be a second or two,” the report said. The AAIB concluded that this momentary distraction was a factor in the subsequent upset.
“The upper winds information provided on the device was pertinent to the flight but was not as important at this point as the flight path and energy information available on the PFD,” the report said. “Therefore, he did not recognise the alerting features of low energy flight, specifically an unusually nose-high attitude and unusually low, and decreasing, airspeed.”
The pilot told investigators that he was “head down,” looking at the PED when he heard a “click,” which likely was the autopilot automatically disengaging at design limits, and felt the Citation pitch nose-down and roll right.
“The pilot then recalled a violent and very confusing rolling departure from controlled flight,” the report said. “The aircraft almost immediately entered high cirrus cloud, obscuring the horizon.”
The pilot was unable to interpret the information that was being presented on his PFD; he said it was information that he had not seen before. He recalled that the PFD on the right side of the panel was presenting similar information. The report noted that, as a designed result of the upset, the PFDs had automatically “decluttered,” eliminating information that was not pertinent to recovery from the unusual attitude.
“The pilot made several attempts to recover the aircraft [to controlled flight], although he could not later recall what control inputs he made,” the report said. “He recalled selecting idle thrust and achieving almost level flight at one point, but he had not increased thrust, and the aircraft slowed rapidly and again departed from controlled flight.”
The Citation eventually entered clear air as it again descended out of control. With a visible external horizon, the pilot was able to regain control. He re-established communication with air traffic control (ATC) and was told to maintain FL 280.
After leveling off at that altitude, the pilot released the control yoke, believing that he had re-engaged the autopilot. However, the aircraft immediately pitched nose-up and climbed about 2,000 ft before the pilot again regained control. He then noticed that the autopilot was not engaged and that the pitch trim was positioned full nose-up.
“Having set an appropriate trim position and now in a stable flight regime, the pilot confirmed that his passenger was uninjured,” the report said. The three small dogs also had escaped injury.
The pilot noticed that the upper surface of the left wing was deformed, and the passenger confirmed that there was similar damage to the right wing. The Citation was over Coventry at the time, and the pilot decided to return to Leeds Bradford Airport because he was familiar with the airport and knew the weather conditions there. Due to the apparent wing damage, he decided not to exceed 1,000 fpm during the descent.
“He informed ATC of his decision, and the remainder of the flight was without further incident,” the report said. “The pilot stated that the aircraft handling appeared unchanged by the damage it had sustained.”
The recorded data indicated that the pitch attitude was 11.5 degrees nose-up when the aircraft stalled. It then banked steeply right, pitched 9 degrees nose-down, banked steeply left and right, and then completed five 360-degree rolls to the right.
“The derived roll rates increased progressively from 111 degrees/second to 120, 152, 153 and finally to 181 degrees/second,” the report said.
Examination of the Citation revealed that the wings had been damaged substantially by overload forces during the upset. Five ribs in the outboard sections of both wings were buckled, and the bonded joints between the ribs and the wing skins had failed.
“The upper and lower outboard wing skins of both wings were permanently deformed, with a significant loss of aerofoil shape,” the report said. “The damage was consistent with symmetrical ‘pullout’ manoeuvre loads between +3.6 g (‘limit’ load) and +5.4 g (‘ultimate’ load).
“Despite the disruption to the wing structure, which on this aircraft type forms an integral fuel tank, no fuel had leaked from the wings and the wing skins remained firmly attached to the front and rear spars.”
Investigators found that both ailerons and the horizontal stabilizer also had been damaged during the upset.
The EGPWS had generated 13 bank angle warnings. However, the AReS data revealed that the Citation’s angle-of-attack (AOA) sensing system vane had become “stuck” as the aircraft neared the assigned cruise altitude and, as a result, the stall warning (stick shaker) system had not activated before the stall occurred. The data indicated that this also had happened on a previous flight.
Tests of the AOA vane, manufactured by Safe Flight Instrument Corp., showed that its heating system functioned properly. However, X-ray scans of the vane case revealed that an internal seal had become displaced, which could have allowed moisture to enter the case when the aircraft was parked and unpressurized.
“There was, however, no visible evidence of water staining or corrosion on the internal components of the AOA case when those parts were examined, but it is likely that only a small amount of moisture would be required to create an intermittent AOA ‘sticking’ mechanism,” the report said. “The sticking of the AOA position represented a subtle, dormant failure that was not readily apparent to the pilot, as it did not generate a warning annunciation.”
Based on these findings, the AAIB recommended that the U.S. Federal Aviation Administration require Cessna Aircraft to collect recorded flight data that would show the frequency at which Safe Flight Model C-12717-1 AOA vanes become stuck and to use the data to conduct a safety assessment of the Citation CJ2’s stall warning system.
In a concluding statement about the accident, the AAIB report said, “The pilot operated the aircraft in an autopilot mode which left it vulnerable to a stall and did not monitor the reducing airspeed as the aircraft reached its cruising altitude. The ‘sticking’ of the stall warning system removed the safety feature specifically designed to protect against this.”
This article is based on “AAIB Bulletin 1/2015” report no. EW/C2013/12/05.
Featured image: © Chris Procter | AirTeamImages
Flight path: U.K. Air Accidents Investigation Branch; map base, Susan Reed and Nilfanion | Wikimedia CC-BY-SA 3.0