07-31-2017, 02:03 PM
Our very own ATSB gets a mention in BCA magazine.
This article appears in the July 2017 issue of Business & Commercial Aviation with the title “Unstabilized Approach?”
A Malaysia Airlines Airbus A330 began its approach to Melbourne (Tullamarine) Airport at 0750 Eastern Summer Time on March 14, 2015, after an uneventful flight from Kuala Lumpur. In the final stages of the approach, at approximately 50 ft. AGL, the captain reported feeling the aircraft sink and manually increased the thrust to slow the rate of descent. Despite this action, the aircraft made a hard landing — hard enough to require replacement of the aircraft’s main landing gear. There were no reported injuries.
The Australian Transportation Safety Board (ATSB) mounted a major investigation into the incident. Ultimately, the safety board determined that the final approach had become unstable because of the pilot flying’s control inputs after he disengaged the autopilot (approximately 700 ft. AGL) and descended below the desired vertical profile. “The continuation of the approach and an inappropriate attempt to recover the situation,” said the board, “led to a high rate of descent at touchdown.”
Figure 1: Melbourne airport is equipped with a precision approach indicator (PAPI) system. The system provides a method for pilots to maintain the 3-deg. glide path by referencing the color of the four lights. The crew of an aircraft on the 3-deg. glide path would see two white lights and two red lights. Credit: Flight Safety Australia
Soon after the event, Malaysia Airlines circulated a memorandum to its A330 flight crews highlighting the incident and advising of the relevant procedures intended to minimize the chances of a similar occurrence. The flight crew involved also undertook additional training and assessment before returning to flight duties.
Perhaps more important is the ATSB’s message to the flying community at large: “A stable approach significantly reduces the risk of a hard landing. If an approach does become unstable, a rushed attempt to recover the approach may produce an undesirable aircraft response. There is also a risk of breaking down the shared understanding between the pilots, which in turn limits the opportunity of the other flight crew to detect or react to inappropriate actions. When landing, pilots should maintain a safety philosophy of ‘if in doubt, go around.’”
What Happened
The incident occurred in the late stages of the Airbus’ approach to Runway 34 at Melbourne. The flight was a regular passenger service from Malaysia. Bureau of Meteorology weather data was consistent with the flight crew’s reports of fine flying conditions and not a factor in the incident. The flight crew consisted of a captain, who was the pilot flying (PF) and a first officer, who was the pilot monitoring (PM).
Melbourne’s Runway 34 was not equipped with an ILS, so a non-precision approach was performed. The runway did have a precision approach path indicator (PAPI) system consisting of four lights that provided visual guidance for the pilots to determine if the aircraft was maintaining the correct glidepath. Figure 1 shows what a crew using the PAPI system would expect to see depending on their relative height to the correct approach path. A crew maintaining the nominal 3-deg. path should see two white lights and two red lights.
Flight data showed that the Airbus pilot’s sidestick pitch control inputs increased in frequency and magnitude after he disengaged the autopilot.
In response to these inputs, the aircraft’s autothrottle system varied the engine thrust to maintain a target speed, as per system design, and the aircraft pitch angles fluctuated between approximately -0.5-deg. nose down and +5.0-deg. nose up. The net result of the varying thrust settings and pitch angles was a fluctuating rate of descent between approximately 380 and 960 fpm.
Figure 2: The ideal glide path during an approach is shown in green, with the PAPI thresholds in red (low) and white (high). The yellow line is the actual path of the aircraft as dervied from the flight data. Just below 150 ft., four red lights would have been indicated to the crew.
Large sidestick inputs, said the Safety Board, specifically nose-down inputs, have the potential to inhibit the vertical speed reduction function, which is an automated function that provides some protection against touchdown at very high vertical speeds.
The airline addresses the use of the A330 autopilot during final approach in its flight crew operating manual (FCOM). The airplane, says the manual, will be stabilized during the final approach when the autopilot is engaged in the proper mode. “Therefore, when disconnecting the AP for a manual landing, the pilot should avoid the temptation to make large inputs on the sidestick,” it cautions.
As the incident aircraft passed 300 ft. AGL, the FDR recorded a rate of descent of 960 fpm. This neared the operator’s maximum stabilized approach limit of 1,000 fpm when below 500 ft. AGL. As well as a high rate of descent, actual exceedences of the operator’s stabilization criteria included:
Large changes to pitch inputted by the pilot flying, including negative pitch values.
Fluctuations in the rate of descent over a large range that were abnormal for the phase of flight.
Incorrect glidepath.
From approximately 250 ft. AGL, the aircraft was trending low and continuing below the glidepath. At this point the pilots would have been able to observe three red lights and one white light on the PAPI. This trend was allowed to continue until the aircraft was well below the desired glidepath to the extent that the PAPI would have indicated four red lights from approximately 125 ft.
Figure 2 displays in green the ideal glidepath during an approach and the PAPI thresholds in red (low) and white (high). The yellow is the actual path of the aircraft derived from the flight data. Just below 150 ft., four red lights would have been indicated to the crew.
The company’s standard operating procedures (SOPs) and manufacturer’s recommendations dictated that if an approach becomes unstable below 500 ft. AGL, a go-around must be initiated by the PF and/or the PM must alert the PF of the unstable approach and encourage a go-around.
Included in the operator’s flight crew training manual and the manufacturer’s operating philosophy of the aircraft were a set of “golden rules” to be followed by flight crews at all times while operating the aircraft. Rule No. 4 states:
If the aircraft does not follow the desired vertical or lateral flight path, or the selected targets, and if the flight crew does not have sufficient time to analyze and solve the situation, the flight crew must immediately take appropriate or required actions, as follows.
The PF should change the level of automation
From managed guidance to selected guidance, or
From selected guidance to manual flying.
The PM should
Communicate with the PF, and when necessary
Challenge the actions of the PF, or
Take over control.
During flight crew interviews there was no indication that the PM communicated with the PF about the unstable approach conditions, considered taking over from the PF and/or encouraged the PF to conduct a go-around, said the ATSB.
Attempted Recovery
At 60 ft. AGL, the captain moved the thrust levers forward momentarily into the TOGA (Takeoff/Go Around) detent. He told investigators that this was done in response to a feeling that the aircraft was sinking below the path, and the intention was to reduce this sink by applying more thrust to the engines. The placement of the thrust lever into TOGA put the aircraft automation into the go-around autopilot modes and changed the display on the primary flight display flight mode annunciator (FMA). The captain subsequently reduced the thrust levers.
The PM, upon seeing the modes on the FMA change, assumed that a go-around was being conducted and awaited further announcement from the captain. The PM reported noticing the thrust levers reducing and that the aircraft was not flaring and applied nose-up input to the sidestick at the same time as the PF.
At the time of the occurrence there was no procedure on the A330 for the use of manual thrust for this purpose. Prior to 2009, a procedure existed in the A330 documentation that allowed a specific use of manual thrust in difficult environmental conditions. Airbus advised that after an analysis of in-service events the procedure was removed from all operational documents. An FCOM bulletin was published at the time, which explained the removal of the procedure.
Manipulation of the thrust levers in this manner had the effect of:
Disengaging the autothrottle, thereby inhibiting some of the available autoflight system protections including the target speed function (which automatically maintained the desired speed and adjusted for fluctuation in the wind), and the vertical speed reduction function.
Causing a pitch-up tendency — as underslung engines increase thrust, they typically apply an upward pitching moment to the aircraft. This usually requires a large nose-down input by the pilot flying to prevent the pitch-up. At this stage in the approach applying large nose-down inputs differed from the gradual nose-up input normally required to complete the flare.
Breaking down of the shared mental model between the pilot flying and pilot monitoring in that the latter believed the advancement of the thrust levers was the initiation of a go-around.
Shortly after the power changes, the aircraft touched down at a vertical speed of approximately 700 fpm and with a vertical acceleration of 2.61g. The aircraft also touched down at approximately 170 m (560 ft.) from the landing threshold, short of the normal touchdown zone of between 304 m (1,000 ft.) and 609 m (2,000 ft.).
Airbus produced a complete load analysis of the event based on recorded flight data provided by the operator. The conclusion of this analysis was that several components in the left and right gear had “exceeded their design load limits and may have been detrimentally overloaded.”
The manufacturer’s analysis assessed the affected components to be unserviceable and requiring replacement before the next flight. Several supplementary inspections were also required.
Safety Analysis
In its analysis of the event, the ATSB said a stable approach is a crucial element of final approach. Identifying an unstable approach and taking the appropriate corrective action is key to maintaining safety at touchdown. This analysis examines the stability of the aircraft and the decision-making in the final stages of the approach.
Analysis of the flight data in this incident relating to the stability of the final approach showed from 300 ft. AGL the approach was unstable due to large control inputs, erratic rates of descent and deviation below the glidepath. Throughout the final approach, the PF’s sidestick inputs determined the aircraft’s vertical profile and, in the absence of any external factors, were therefore contributing to the unstable approach.
Researchers have stated that, in time-constrained environments, individuals can make decisions using intuitive reasoning where the steps are often unconscious and based on pattern recognition. For intuitive decision-making, an experienced individual will identify a problem situation as similar to a situation they have dealt with before and will extract a plan of action from memory. If time permits, they will confirm their expectations prior to initiating action. If time does not permit, actions will be initiated with uncertainty that may result in a poor decision.
In this investigation, said the safety board, the PM was late to recognize that the approach was unstable and as such did not encourage the PF to conduct a go-around, as per the operator’s SOPs.
During the last 50 ft. of the approach (4 sec. before touchdown) the PF inappropriately used the thrust levers in an attempt to arrest the high rate of descent. Despite the absence of the standard phraseology of “go-around” by the PF, the PM mistook this advancement of the thrust levers as the initiation of a go-around. The PM, expecting that the PF had initiated a go-around and realizing that the aircraft was still descending with a nose-down attitude, placed a hand on the sidestick and applied a nose-up input. A dual input was therefore recorded as the aircraft touched down.
“Probably due to time constraints, neither crewmember communicated their intentions,” said the safety board. “Neither the initial thrust advancement nor the subsequent thrust reversal by the PF were communicated to the PM. As a result, the PM was unclear about the action taken by the PF.”
Large and erratic pitch inputs by the PF, as well as large fluctuations in the rate of descent and visual reference of the PAPI lights provided opportunities for the crew to recognize an unstable approach, said the Board. Despite this, there was no evidence of actions or support language to suggest that the unstable approach was identified. The operator’s procedures whereby an unstable approach should result in a go-around were not followed.
The manual thrust technique used by the captain to arrest the sink and recover the approach was used on other aircraft types previously flown by the captain. There was no current approved procedure on the A330 for this technique. However, analysis of the flight data determined that this action alone did not contribute to or increase the severity of the hard landing.
The captain and first officer told investigators that, in retrospect, they should have conducted a go-around in accordance with the operator’s training procedures. A Flight Safety Foundation study of hard landings determined — statistically, anyway — that runway contact from a late go-around is preferable to attempting to recover an unstable approach.
Elements of an unstable approach are not unusual during flight operations. However, the actions taken by the flight crew in response are key to maintaining flight safety, said the ATSB. “The training records of the crew were reviewed to establish the possibility that a training or performance issue led to the PF’s actions. Apart from the PF’s misunderstanding of the use of the thrust levers to reduce the rate of descent, there was no indication of a systemic issue with either crewmember.”
The ATSB made these formal findings of “contributing factors” after its investigation:
The final approach became unstable at around 300 ft. above the ground due to the control inputs from the captain.
Inadequate monitoring and communication by the crew led to a lack of recognition of the undesirable flight state and the continuation of an unstable approach.
Continuation of the unstable approach led to a high rate of descent at touchdown and resulted in a hard landing in excess of the aircraft design loads and short of the normal touchdown area.
The captain used an unapproved manual thrust procedure in an attempt to recover the approach.
The bottom line, as expressed so elegantly by the ATSB: When in doubt, take it around!
This article appears in the July 2017 issue of Business & Commercial Aviation with the title “Unstabilized Approach?”
A Malaysia Airlines Airbus A330 began its approach to Melbourne (Tullamarine) Airport at 0750 Eastern Summer Time on March 14, 2015, after an uneventful flight from Kuala Lumpur. In the final stages of the approach, at approximately 50 ft. AGL, the captain reported feeling the aircraft sink and manually increased the thrust to slow the rate of descent. Despite this action, the aircraft made a hard landing — hard enough to require replacement of the aircraft’s main landing gear. There were no reported injuries.
The Australian Transportation Safety Board (ATSB) mounted a major investigation into the incident. Ultimately, the safety board determined that the final approach had become unstable because of the pilot flying’s control inputs after he disengaged the autopilot (approximately 700 ft. AGL) and descended below the desired vertical profile. “The continuation of the approach and an inappropriate attempt to recover the situation,” said the board, “led to a high rate of descent at touchdown.”
Figure 1: Melbourne airport is equipped with a precision approach indicator (PAPI) system. The system provides a method for pilots to maintain the 3-deg. glide path by referencing the color of the four lights. The crew of an aircraft on the 3-deg. glide path would see two white lights and two red lights. Credit: Flight Safety Australia
Soon after the event, Malaysia Airlines circulated a memorandum to its A330 flight crews highlighting the incident and advising of the relevant procedures intended to minimize the chances of a similar occurrence. The flight crew involved also undertook additional training and assessment before returning to flight duties.
Perhaps more important is the ATSB’s message to the flying community at large: “A stable approach significantly reduces the risk of a hard landing. If an approach does become unstable, a rushed attempt to recover the approach may produce an undesirable aircraft response. There is also a risk of breaking down the shared understanding between the pilots, which in turn limits the opportunity of the other flight crew to detect or react to inappropriate actions. When landing, pilots should maintain a safety philosophy of ‘if in doubt, go around.’”
What Happened
The incident occurred in the late stages of the Airbus’ approach to Runway 34 at Melbourne. The flight was a regular passenger service from Malaysia. Bureau of Meteorology weather data was consistent with the flight crew’s reports of fine flying conditions and not a factor in the incident. The flight crew consisted of a captain, who was the pilot flying (PF) and a first officer, who was the pilot monitoring (PM).
Melbourne’s Runway 34 was not equipped with an ILS, so a non-precision approach was performed. The runway did have a precision approach path indicator (PAPI) system consisting of four lights that provided visual guidance for the pilots to determine if the aircraft was maintaining the correct glidepath. Figure 1 shows what a crew using the PAPI system would expect to see depending on their relative height to the correct approach path. A crew maintaining the nominal 3-deg. path should see two white lights and two red lights.
Flight data showed that the Airbus pilot’s sidestick pitch control inputs increased in frequency and magnitude after he disengaged the autopilot.
In response to these inputs, the aircraft’s autothrottle system varied the engine thrust to maintain a target speed, as per system design, and the aircraft pitch angles fluctuated between approximately -0.5-deg. nose down and +5.0-deg. nose up. The net result of the varying thrust settings and pitch angles was a fluctuating rate of descent between approximately 380 and 960 fpm.
Figure 2: The ideal glide path during an approach is shown in green, with the PAPI thresholds in red (low) and white (high). The yellow line is the actual path of the aircraft as dervied from the flight data. Just below 150 ft., four red lights would have been indicated to the crew.
Large sidestick inputs, said the Safety Board, specifically nose-down inputs, have the potential to inhibit the vertical speed reduction function, which is an automated function that provides some protection against touchdown at very high vertical speeds.
The airline addresses the use of the A330 autopilot during final approach in its flight crew operating manual (FCOM). The airplane, says the manual, will be stabilized during the final approach when the autopilot is engaged in the proper mode. “Therefore, when disconnecting the AP for a manual landing, the pilot should avoid the temptation to make large inputs on the sidestick,” it cautions.
As the incident aircraft passed 300 ft. AGL, the FDR recorded a rate of descent of 960 fpm. This neared the operator’s maximum stabilized approach limit of 1,000 fpm when below 500 ft. AGL. As well as a high rate of descent, actual exceedences of the operator’s stabilization criteria included:
Large changes to pitch inputted by the pilot flying, including negative pitch values.
Fluctuations in the rate of descent over a large range that were abnormal for the phase of flight.
Incorrect glidepath.
From approximately 250 ft. AGL, the aircraft was trending low and continuing below the glidepath. At this point the pilots would have been able to observe three red lights and one white light on the PAPI. This trend was allowed to continue until the aircraft was well below the desired glidepath to the extent that the PAPI would have indicated four red lights from approximately 125 ft.
Figure 2 displays in green the ideal glidepath during an approach and the PAPI thresholds in red (low) and white (high). The yellow is the actual path of the aircraft derived from the flight data. Just below 150 ft., four red lights would have been indicated to the crew.
The company’s standard operating procedures (SOPs) and manufacturer’s recommendations dictated that if an approach becomes unstable below 500 ft. AGL, a go-around must be initiated by the PF and/or the PM must alert the PF of the unstable approach and encourage a go-around.
Included in the operator’s flight crew training manual and the manufacturer’s operating philosophy of the aircraft were a set of “golden rules” to be followed by flight crews at all times while operating the aircraft. Rule No. 4 states:
If the aircraft does not follow the desired vertical or lateral flight path, or the selected targets, and if the flight crew does not have sufficient time to analyze and solve the situation, the flight crew must immediately take appropriate or required actions, as follows.
The PF should change the level of automation
From managed guidance to selected guidance, or
From selected guidance to manual flying.
The PM should
Communicate with the PF, and when necessary
Challenge the actions of the PF, or
Take over control.
During flight crew interviews there was no indication that the PM communicated with the PF about the unstable approach conditions, considered taking over from the PF and/or encouraged the PF to conduct a go-around, said the ATSB.
Attempted Recovery
At 60 ft. AGL, the captain moved the thrust levers forward momentarily into the TOGA (Takeoff/Go Around) detent. He told investigators that this was done in response to a feeling that the aircraft was sinking below the path, and the intention was to reduce this sink by applying more thrust to the engines. The placement of the thrust lever into TOGA put the aircraft automation into the go-around autopilot modes and changed the display on the primary flight display flight mode annunciator (FMA). The captain subsequently reduced the thrust levers.
The PM, upon seeing the modes on the FMA change, assumed that a go-around was being conducted and awaited further announcement from the captain. The PM reported noticing the thrust levers reducing and that the aircraft was not flaring and applied nose-up input to the sidestick at the same time as the PF.
At the time of the occurrence there was no procedure on the A330 for the use of manual thrust for this purpose. Prior to 2009, a procedure existed in the A330 documentation that allowed a specific use of manual thrust in difficult environmental conditions. Airbus advised that after an analysis of in-service events the procedure was removed from all operational documents. An FCOM bulletin was published at the time, which explained the removal of the procedure.
Manipulation of the thrust levers in this manner had the effect of:
Disengaging the autothrottle, thereby inhibiting some of the available autoflight system protections including the target speed function (which automatically maintained the desired speed and adjusted for fluctuation in the wind), and the vertical speed reduction function.
Causing a pitch-up tendency — as underslung engines increase thrust, they typically apply an upward pitching moment to the aircraft. This usually requires a large nose-down input by the pilot flying to prevent the pitch-up. At this stage in the approach applying large nose-down inputs differed from the gradual nose-up input normally required to complete the flare.
Breaking down of the shared mental model between the pilot flying and pilot monitoring in that the latter believed the advancement of the thrust levers was the initiation of a go-around.
Shortly after the power changes, the aircraft touched down at a vertical speed of approximately 700 fpm and with a vertical acceleration of 2.61g. The aircraft also touched down at approximately 170 m (560 ft.) from the landing threshold, short of the normal touchdown zone of between 304 m (1,000 ft.) and 609 m (2,000 ft.).
Airbus produced a complete load analysis of the event based on recorded flight data provided by the operator. The conclusion of this analysis was that several components in the left and right gear had “exceeded their design load limits and may have been detrimentally overloaded.”
The manufacturer’s analysis assessed the affected components to be unserviceable and requiring replacement before the next flight. Several supplementary inspections were also required.
Safety Analysis
In its analysis of the event, the ATSB said a stable approach is a crucial element of final approach. Identifying an unstable approach and taking the appropriate corrective action is key to maintaining safety at touchdown. This analysis examines the stability of the aircraft and the decision-making in the final stages of the approach.
Analysis of the flight data in this incident relating to the stability of the final approach showed from 300 ft. AGL the approach was unstable due to large control inputs, erratic rates of descent and deviation below the glidepath. Throughout the final approach, the PF’s sidestick inputs determined the aircraft’s vertical profile and, in the absence of any external factors, were therefore contributing to the unstable approach.
Researchers have stated that, in time-constrained environments, individuals can make decisions using intuitive reasoning where the steps are often unconscious and based on pattern recognition. For intuitive decision-making, an experienced individual will identify a problem situation as similar to a situation they have dealt with before and will extract a plan of action from memory. If time permits, they will confirm their expectations prior to initiating action. If time does not permit, actions will be initiated with uncertainty that may result in a poor decision.
In this investigation, said the safety board, the PM was late to recognize that the approach was unstable and as such did not encourage the PF to conduct a go-around, as per the operator’s SOPs.
During the last 50 ft. of the approach (4 sec. before touchdown) the PF inappropriately used the thrust levers in an attempt to arrest the high rate of descent. Despite the absence of the standard phraseology of “go-around” by the PF, the PM mistook this advancement of the thrust levers as the initiation of a go-around. The PM, expecting that the PF had initiated a go-around and realizing that the aircraft was still descending with a nose-down attitude, placed a hand on the sidestick and applied a nose-up input. A dual input was therefore recorded as the aircraft touched down.
“Probably due to time constraints, neither crewmember communicated their intentions,” said the safety board. “Neither the initial thrust advancement nor the subsequent thrust reversal by the PF were communicated to the PM. As a result, the PM was unclear about the action taken by the PF.”
Large and erratic pitch inputs by the PF, as well as large fluctuations in the rate of descent and visual reference of the PAPI lights provided opportunities for the crew to recognize an unstable approach, said the Board. Despite this, there was no evidence of actions or support language to suggest that the unstable approach was identified. The operator’s procedures whereby an unstable approach should result in a go-around were not followed.
The manual thrust technique used by the captain to arrest the sink and recover the approach was used on other aircraft types previously flown by the captain. There was no current approved procedure on the A330 for this technique. However, analysis of the flight data determined that this action alone did not contribute to or increase the severity of the hard landing.
The captain and first officer told investigators that, in retrospect, they should have conducted a go-around in accordance with the operator’s training procedures. A Flight Safety Foundation study of hard landings determined — statistically, anyway — that runway contact from a late go-around is preferable to attempting to recover an unstable approach.
Elements of an unstable approach are not unusual during flight operations. However, the actions taken by the flight crew in response are key to maintaining flight safety, said the ATSB. “The training records of the crew were reviewed to establish the possibility that a training or performance issue led to the PF’s actions. Apart from the PF’s misunderstanding of the use of the thrust levers to reduce the rate of descent, there was no indication of a systemic issue with either crewmember.”
The ATSB made these formal findings of “contributing factors” after its investigation:
The final approach became unstable at around 300 ft. above the ground due to the control inputs from the captain.
Inadequate monitoring and communication by the crew led to a lack of recognition of the undesirable flight state and the continuation of an unstable approach.
Continuation of the unstable approach led to a high rate of descent at touchdown and resulted in a hard landing in excess of the aircraft design loads and short of the normal touchdown area.
The captain used an unapproved manual thrust procedure in an attempt to recover the approach.
The bottom line, as expressed so elegantly by the ATSB: When in doubt, take it around!