The following is a summary of a crosswind landing incident to an A340 landing at Melbourne, Australia. As an observer I find myself thinking that there is some thing a little more basic going on than the often said "follow SOP's" or "more training required" in dealing with the Airbus philosophy.
Full report at
http://www.atsb.gov.au/publications/...505311_001.pdf
At 1200 Eastern Standard Time on 26 October 2005, the outboard bead heel of the number-1 wheel tyre on the left main landing gear (MLG) of an Airbus A340-642 (A340) aircraft, registered HS-TNA, separated from the outboard rim of the wheel assembly during a landing on runway 16 at Melbourne Airport, Vic. The landing was conducted during gusting crosswind conditions.
The number-1 wheel tyre deflated immediately after the bead heel separated from the wheel rim. The tyre then partially disintegrated during the remainder of the landing roll, and the tyre tread detached from the tyre casing. Following the number-1 wheel tyre deflation, the crew maintained control of the aircraft and, apart from some minor deviations to the left and right of the runway centreline, tracked along the centreline.
The aircraft touched down with 15-degrees of yaw as a result of its handling by the flight crew. That yaw angle was greater than recommended by the aircraft manufacturer, and increased the risk of damage to the MLG at touchdown. It also increased the risk that the resultant groundslip angle of the MLG tyres would exceed the ‘saturation’ point at which they entered a fully-skidded state.
The pilot in command made dual side stick inputs during the latter stages of the approach intending to assist the copilot to maintain the attitude and trajectory of the aircraft. Those dual inputs compounded the handling difficulties being experienced by the copilot and increased the associated risks. Those risks could have been mitigated by the pilot in command taking control of the aircraft and pressing the side stick priority pushbutton at the point where he appeared to have become concerned about its attitude and trajectory, instead of making dual side stick inputs.
Side stick operation – dual inputs
The operator’s A340 FCOM also contained information that the PNF should not make control inputs to correct the handling of the aircraft by the PF. The lack of a direct mechanical linkage between the side stick controllers meant that there was no tactile feedback provided to the PF if the PNF was making concurrent or dual side stick inputs. (The aircraft manufacturer indicated that the interconnection of the aircraft’s side sticks ‘would be operationally not beneficial and technically not efficient’)
The aircraft was, however, fitted with a warning system to alert the crew of simultaneous inputs on both side sticks in the event that the side stick priority system was not activated. A 2-degree deflection of the PNF’s side stick in any direction from the neutral position resulted in the illumination of the green SIDE STICK PRIORITY warning lights on the glareshield in front of each crew member. In addition, the ‘DUAL INPUT’ synthetic voice message was activated by the flight warning system.
The aircraft manufacturer has examined the reasons for dual sidestick inputs during line operations. Advice received from the manufacturer included that, in normal flight conditions, the practice should not occur if ‘…proper airmanship and CRM [crew resource management] applied.’
An analysis by the manufacturer of reported instances of dual sidestick inputs has revealed they may be ‘spurious’, ‘comfort’, or ‘instinctive’ interventions on the part of the PNF. Those inputs can be defined as follows:
•
Spurious dual inputs. Spurious dual inputs are unintentional, are of short-term duration and small in magnitude, and result in only marginal effects on an aircraft’s pitch and roll.
•
Comfort dual inputs. Comfort dual inputs are intentional, short-term interventions by the PNF. The intention of the PNF is to correct or improve the aircraft’s attitude or trajectory during a precision manoeuvre, such as a landing approach or landing flare. Comfort inputs are normally small deflections, and may be the same as, or opposite to the PF’s sidestick inputs. They usually result in only minor effects on an aircraft’s altitude and/or trajectory, and are ‘…thus in most cases unnecessary.’ In most cases, the PF was unaware of any ‘comfort’ inputs by the PNF.
•
Instinctive dual inputs. Instinctive dual inputs are ‘reflex’ interventions by the PNF, acting out of surprise at some unexpected event that may occur during a dynamic flight manoeuvre, such as the landing flare. Those interventions are significant in terms of stick deflection, and are usually initially in the same direction as the PF’s stick inputs. They have the potential to affect an aircraft’s behaviour, and may lead to over-control of an aircraft. As with comfort dual inputs, in most cases the PF is unaware of any ‘instinctive’ dual inputs by the PNF.
Previous incidents involving dual sidestick inputs on ‘fly-by-wire’ aircraft
The investigation examined three reports on previous dual sidestick input occurrences involving Airbus fly-by-wire aircraft types. Those reports revealed that,
because neither crew member is provided tactile feedback of any sidestick inputs made by the other, dual sidestick inputs are problematic. (My bolding)
On 21 June 1996, the crew of an Airbus A340 aircraft enroute from Dallas/Fort Worth Airport to Houston Intercontinental Airport received a “descend” resolution advisory from the aircraft traffic collision avoidance system. The copilot was the PF. The US National Transportation Safety Board (NTSB) factual report19 on the incident revealed that:
The Captain initiated an immediate descent. The Captain did not make a verbal announcement that he was taking command of the left side stick control.
In that incident, the dual inputs from the pilot in command’s and copilot’s side stick controllers continued until the copilot noticed that the pilot in command was providing sidestick input, and returned the right side stick controller to the neutral position.
At the time of that occurrence, the ‘DUAL INPUT’ side stick warning system was not available on A340 aircraft.
On 21 June 2000, an Airbus A321 aircraft was involved in a tailstrike accident during a landing at London Heathrow. The copilot was the PF. The UK Air Accidents Investigation Branch (AAIB) investigation report20 included information that:
The aircraft touched down at an airspeed of 130 kt CAS, with a pitch attitude of 7.4° nose-up and a normal acceleration of 2.0g. The FO's sidestick position was 92.5% nose-up demand with an upelevator angle of 12.3°. The FO's sidestick demand then reduced, towards 46.8% nose-up demand.
The ground spoilers deployed automatically; this is designed to occur when both the main landing gear oleo switches are compressed. The FDR showed that these switches then 'unmade' indicating that the aircraft had rebounded into the air. The pitch attitude continued to increase to a maximum of 9.8° nose-up, which was reached just as the aircraft mainwheels touched the ground again. The tailscrape occurred at this point. The second touchdown recorded a normal acceleration of 1.6g at which time the commander's sidestick moved forward to a 56.3% nose-down demand.
The analysis section of the AAIB investigation included information that:
The commander did not anticipate a problem until after the aircraft's initial touchdown. He could not have been aware of the control inputs applied by the FO21, in particular the continued aft sidestick input late in the landing, because his own sidestick showed no movement.
and that:
The sidestick control authority logic requires a different method of intervention by commanders from that which they may have experienced on other aircraft types.
Because of the difficulty of detecting the inputs made by the other pilot (My bolding) early takeover of control based on flight characteristics is required.
On 9 October, 2000, another Airbus A321 aircraft was involved in another tailstrike accident during a landing at London Heathrow. The copilot was the PF. The AAIB investigation revealed that, during the latter stages of the landing approach, the pilot in command became concerned at the high rate of descent. The pilot in command then applied aft sidestick, which ‘progressed to nearly full aft sidestick by a height of 10 feet, in order to arrest the rate of descent but he did not activate his sidestick takeover push button.’
The AAIB investigation report22 included a conclusion that:
As with other such incidents the commander could not see the control inputs of the FO and his first indication was a high rate of descent at about 40 feet shortly after the flare was initiated. He did not activate his sidestick takeover button and, given the circumstances, this action would not have prevented the tail of the aircraft contacting the runway. This occurred following the bounce on the second touch down when the commander was using aft sidestick to prevent the nose wheel coming down heavily.
During the latter stages of the landing approach, the pilot in command, who was the pilot not flying (PNF), commenced dual sidestick control inputs. Those dual sidestick inputs contravened the instructions relating to the operation of the aircraft’s fly-by-wire and side stick systems, which were contained in the operator’s A340 Flight Crew Operating Manual (FCOM).
The passage of the strong westerly winds over the Box Forest area to the north-west of Melbourne Airport would probably also have resulted in lee turbulence in the vicinity of the touchdown zone of runway 16. That turbulence was likely to have contributed to the aircraft’s unexpected roll deviations, including the recorded sudden roll to the left shortly after the aircraft passed through a height of about 70 ft. The pilot in command’s dual sidestick input at that point seemed to have been a ‘comfort’ intervention to correct the aircraft’s attitude or trajectory at that stage of the approach.
The pilot in command’s dual sidestick interventions became more significant in terms of stick deflection as the aircraft neared the ground, and were in mostly the same direction as the copilot’s stick inputs. Those dual sidestick would have provided ‘global’ electronic demand ‘orders’ to the flight control computers that were greater than those ‘ordered’ by the copilot. Consequently, the aircraft’s response to those ‘global’ demand ‘orders’ would likely have been greater-than-expected by the copilot. Moreover, the unexpected magnitude of aircraft’s response seemed to result in both crew members applying opposite sidestick inputs to counteract that unexpected response, leading to an over-control of the aircraft.
The investigation was unable to determine whether the copilot was aware of the pilot in command’s dual sidestick inputs, even though they resulted in aural ‘DUAL INPUT’ synthetic voice messages from the flight warning system (FWS). It was likely that, during the latter stages of the approach, the copilot’s attention was focussed on the external visual cues in order to maintain the aircraft tracking on the extended centreline of the runway in the gusting crosswind conditions. In addition, the copilot’s attention seemed also to have been focused on countering the unexpected magnitude of the aircraft’s roll and pitch that resulted from the ‘global’ demand ‘orders’. The copilot’s focus on correcting the aircraft’s attitude and trajectory, together with the numerous FWS synthetic voice messages, may have resulted in the copilot not comprehending the significance of the aural ‘DUAL INPUT’ warnings, and that they were a cue to the reason for the aircraft’s unexpected handling response.
The application of two-thirds full right rudder during the landing flare increased the yaw angle to more than that recommended by the manufacturer for a ‘…a safe crosswind landing…’ where the crosswind component exceeded 15 to 20 kts. The right yaw resulting from the application of that rudder would have increased the aircraft roll to the right, and therefore increased the risk of a right wingtip or wing-mounted engine pod strike. The right roll was countered by rapid and large left sidestick inputs by both crew members that resulted in an effective full left sidestick input in what appeared to be an instinctive ‘reflex’ response by both crew members to prevent that risk.
There was no attempt by either crew member to partially ‘decrab’ the aircraft during the landing flare. That was contrary to the advice provided by the manufacturer for the performance of ‘…a safe crosswind landing…’ in higher crosswind conditions. The lack of any ‘decrab’ therefore decreased safety margins and increased the risk of main landing gear (MLG) damage at touchdown. It also increased the risk that the resultant groundslip angle at touchdown would be of sufficient magnitude that the MLG tyre side forces exceeded the ‘saturation’ point at which they entered a fully-skidded state.