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ASN Wikibase Occurrence # 245351
Last updated: 16 January 2021
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Time:14:22 UTC
Type:Silhouette image of generic C525 model; specific model in this crash may look slightly different
Cessna 525 Citation CJ1+
Registration: N680KH
C/n / msn: 525-0680
Fatalities:Fatalities: 0 / Occupants: 4
Other fatalities:0
Aircraft damage: None
Category:Serious incident
Location:Bournemouth Airport -   United Kingdom
Phase: Initial climb
Departure airport:Bournemouth International Airport (BOH/EGHH)
Destination airport:Rotterdam/The Hague Airport (RTM/EHRD)
Investigating agency: AAIB
The pilot had recently bought the aircraft, a Cessna 525 Citation CJ1+, which had been fitted during its previous ownership with a Tamarack ATLAS (Active Technology Load Alleviation System) wing extensions and winglets.
Active aerodynamic control surfaces are positioned in the horizontal section of these extensions. These control surfaces (known as Tamarack Active Camber Surfaces, or TACS) are automatically activated in high positive or negative g situations to unload the wing and keep the wing loading within the original envelope.

On the incident day, the owner was intending to fly himself and three friends from Bournemouth to Rotterdam. The pilot, who operated the aircraft in a single pilot capacity, occupied the front left seat. One friend occupied the front right seat, and the other two were seated in the passenger cabin.
The aircraft took off from runway 08 at Bournemouth Airport at 14:17 and the pilot engaged the autopilot shortly afterwards. It flew a heading of 075° and climbed to altitude 3,000 ft. At 14:18:35 hrs ATC instructed the aircraft to climb to FL100, which the pilot read back. The ATCO instructed '...resume own navigation direct goodwood’, which required the aircraft to turn right.
No response was received from the pilot to that and two further transmissions.
The pilot recalled feeling light vibration, then a button on the left of the instrument panel labelled ‘atlas’ illuminated, displaying the text ‘atlas inop limit 140 kias’ in red.
At 14:18:39 hrs, when the aircraft was around 6 nm east of the airport, at 3,000 ft amsl and 258 KIAS, the aircraft rolled left with a rate the pilot described as "very quick". As it rolled through 45° the autopilot disengaged automatically.
The pilot reported applying full right aileron and full right rudder, but these actions were insufficient to control the aircraft. He moved the throttles to idle and used both hands on the control column, but the aircraft continued descending. Recorded data showed that a bank angle alert was generated at around 60° roll, and there was a sharp increase in normal acceleration, which reached +2.65 g. The aircraft’s roll angle peaked at 75° left wing down, with 9° nose down pitch, 19 seconds after the onset of the roll. Its rate of descent peaked soon after at 4,500 ft/min, corresponding with an airspeed of 235 KIAS, reaching a minimum altitude of 2,300 ft.
During the upset the pilot pressed the illuminated ATLAS button and re-set the atlas main circuit breaker (CB), but neither action had an effect.
At 14:19:18 hrs the pilot reported ‘a problem’ to ATC. The ATCO attempted to ascertain what was wrong but the pilot sounded breathless and strained, and his transmissions were incomplete and difficult to decipher. Whilst the pilot did not declare an emergency the ATCO, believing he sounded "extremely shaken", advised him to join left hand downwind for runway 08, and instigated a full emergency procedure.
The pilot recalled it took all his strength to lift the aircraft's nose, reduce its airspeed, and recover the bank angle to around 30° left wing down. He climbed the aircraft to 3,200 ft and its airspeed reduced to 144 KIAS. It then entered a descending left turn.
After descending from 900 ft to 300 ft amsl during the downwind leg, the aircraft turned on to base leg above a sports field. The pilot reported using continuous full right aileron and some right rudder until landing. Less right rudder was required as airspeed reduced, and he achieved lateral control by modulating his right foot pressure. By reducing that pressure the aircraft turned continuously through left base on to a 1 nm final approach, right of the runway centreline at 200 ft amsl. The tower controller described the turn as so tight that the aircraft appeared to be "on its side". He and several colleagues believed the aircraft would crash short of the airfield. At the landing speed of 105 KIAS the pilot believed he could land the aircraft straight so used "less right foot" to straighten the approach and, when over the runway, applied full flap. The aircraft landed at 14:23 hrs.

During the investigation of the TACS Control Unit (TCU) a screw and washer which attached and earthed the electrical connector printed circuit board to the unit’s chassis were missing and found elsewhere in the unit. These items caused a short circuit in the TCU.

On April 23, 2019, EASA issued an emergency airworthiness directive requiring Tamarack ATLAS to be deactivated and the TACS to be fixed in place on ATLAS-equipped Citations.
Tamarack Aerospace came with a fix, resolving the EAD on 11 July 2019.

AAIB Conclusion:
The uncommanded left roll occurred because a short circuit in the left ATLAS Control Unit caused the associated control surface to fail in the fully deflected up position.
The pilot, who had recently purchased the aircraft already modified with the ATLAS winglets, was not aware of the associated aircraft flight manual supplement, which was absent from the relevant section of his aircraft’s flight manual.
The pilot’s instinctive response to the aircraft upset was different to that assumed by certification flight testing and the ATLAS inoperative emergency procedure. Some of those differences may be addressed by the ‘Aircraft Safety and Certification Reform Act of 2020’ which is underway in the USA.



23 April 2019: Upset incidents lead EASA to issue emergency AD on CitationJets with active winglets

Accident investigation:
Investigating agency: AAIB
Status: Investigation completed
Duration: 1 year and 7 months
Download report: Final report


Photo of N680KH courtesy

Bournemouth - International (EGHH / BOH)
15 August 2019; (c) Howard J Curtis

TACS Control Unit (TCU) showing where the crew should have been, and where it was found (AAIB)

Revision history:

03-Dec-2020 20:44 harro Added
03-Dec-2020 21:07 harro Updated [Source, Photo]
04-Dec-2020 20:20 harro Updated [Narrative]
04-Dec-2020 20:39 harro Updated [Narrative]

Corrections or additions? ... Edit this accident description