Photo: UAE 521 flightpath and impact along Runway 12L, AAIS, Figure 1
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The Aircraft, General
This airplane had all the necessary "bells and whistles" to fly its assigned task.
- The Aircraft was certified, equipped, and maintained in accordance with the Civil Aviation Regulations of the United Arab Emirates.
- The Aircraft was airworthy when dispatched for the flight, and there was no evidence of any defect or malfunction that could have contributed to the Accident.
- The Aircraft was structurally intact prior to impact, and mostly remained intact after the impact.
- The post-Accident examination reports did not find any evidence of any Aircraft component or system malfunctions.
- The Aircraft was equipped with a long landing alerting system that annunciated a ‘long landing’ alert at a distance of 1,280 m beyond the runway 12L threshold, some 92 m further along the runway than the programmed alert distance.
- The A/T provides thrust control from takeoff to landing. The selective positions of the A/T are: ‘off’ and ‘arm’. When the A/T is armed and activated (by the pushbutton switch), it will move the thrust levers to achieve either ‘speed’ or ‘thrust’ control, depending on the selected mode.
- Either pilot can move the thrust levers manually at any time to override the A/T. After manual positioning, the A/T will return the thrust levers to the position corresponding to the previously active mode. With the A/T active, during landing and below 25 ft radio altitude, the thrust levers will move towards the idle position and the A/T mode on the FMA will change to ‘IDLE’.
- After touchdown, in manual or automatic flight, until the thrust reversers are selected, the A/T mode stays at ‘IDLE’. By design, because the TO/GA switches are now inhibited, either because the weight-on-wheels is valid, or the aircraft radio altitude is less than two feet for more than three seconds, pushing the TO/GA switch does not affect the mode and the A/T will stay at ‘IDLE’.
- In order to activate the A/T for a go-around, all of the following conditions must be fulfilled: A/T arm switch is in the ‘arm’ position; aircraft is in ‘air’ mode; glideslope is engaged or the flap lever is not in the ‘up’ position; thrust limit mode is not at ‘takeoff’; and either TO/GA switch is pushed.
- Pushing either TO/GA switch when the A/T is enabled will automatically advance the thrust levers to the go-around thrust setting. If the TO/GA switch is pushed during the inhibited period, the A/T will not automatically advance the thrust levers to the go-around thrust setting.
- The flight director does not provide go-around guidance when the aircraft is on the ground. The flight director is enabled when the aircraft is in ‘air’ mode and upon pushing a TO/GA switch when glideslope mode is engaged or when the flaps are out of the ‘UP’ position.
Main landing gear truck tilt/untilt
Photo: Main landing gear tilt position, AAIS, figure 7.
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- Each main landing gear consists of a six-wheel truck with a truck position actuator that tilts the truck up 13 degrees in preparation for landing and down 5 degrees for gear retraction.
- When the main gear is extended for landing, the ‘tilt’ position of the truck allows the aft wheels to contact the runway first. The truck will then ‘untilt’ allowing the center and forward wheels to contact the runway surface.
- Truck tilt sensors are installed to sense the tilt position of the truck. From 13 to 10 degrees, the sensor transitions from the ‘tilt’ to the ‘untilt’ position.
Air/ground system (AGS)
- The AGS transmits an ‘air’ or ‘ground’ mode signal to aircraft systems including the A/T, auto speedbrake, and autobrake systems.
- Two variable reluctance strain load sensors are installed on each wing side landing gear beam. In ‘ground’ mode, the aircraft weight-on-wheels (WOW) will load the landing gear beam causing it to elastically bend.
[AAIS, §22.214.171.124] The FDR recorded parameter for the main gear truck position of TILT / UNTILT is the truck tilt sensor. The main gear WOW sensor state is not a recorded FDR parameter. For a period of six seconds, from 0837:16 to 0837:22, the main landing gear entered a series of ‘tilt’ (‘air’ mode) and ‘untilt’ (‘ground’ [GND] mode) cycles. Some cycles were in phase, or partially in phase, for both main gear, while for other cycles the main gear attitude changed for a single gear only.
Photo: Calculated go-around after TO/GA switch push, AAIS, figure 21.
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- The evaluation showed that maintaining the same pitch attitude as the Accident flight, eight seconds after the Commander called for a ‘go-around’, pushing the TO/GA switch at 13,385 seconds FDR time, which corresponds to the time that the landing gear lever was moved to ’up’, the Aircraft could have achieved a positive rate of climb as the engine thrust increased. After the TO/GA switch push, the A/T would have moved the thrust levers with a maximum rate of 10.5 degrees per second, to achieve an EPR of 1.414 in approximately eight seconds. During this time, the Aircraft would have descended and the minimum radio altitude reached by the Aircraft to safely fly away in the hypothetical scenario was 17 ft. Figure 21 illustrates the calculated simulation.
- For UAE521, the actual thrust lever increase, which was a manual advancement by the Commander at 13,392 seconds FDR time, was performed fifteen seconds after the Commander called for a ‘go-around’.
- During the manual landing, the FCTM ̶ Flare and Touchdown, states that after the aircraft nose passes the threshold, the pilot is recommended to change his visual sighting point to the far end of the runway in order to control the pitch attitude during the flare. The FCTM recommends initiation of the flare when the main gear is approximately 20 to 30 ft above the runway by increasing the pitch angle approximately 2 to 3 degrees in order to slow the rate of descent.
- For airspeed control, the FCTM states that when the A/T is engaged and active, the thrust levers are automatically retarded to idle at 25 ft radio altitude and the 5 kt addition to VREF will be bled off and the engine thrust will reach idle power at touchdown.
- The FCTM stated that small pitch adjustments will be required in order to maintain the desired descent rate after flare initiation and to hold sufficient back pressure on the control column to keep the pitch attitude constant.
I used to preach this in the Boeing 747 and continue to do so in every Gulfstreams I've ever flown: you need to keep a descent all the way to touchdown, I shoot for 100 to 200 fpm. The ground effect off the wings and compression of the struts takes care of softness of the touchdown. This from someone with a lot of hours in the Boeing 777:
The most common way to lose a smooth touchdown (not that smooth touchdowns mean its a good landing) is that the aircraft is ‘held-off’ just as it touches down. By that, I mean not letting the aircraft weight settle into the oleos. If you hold it off, just as the wheels touch the runway, the trucks will go off tilt (13-14 degrees) and the spoilers will deploy and the weight will come down rapidly onto the oleos with a tiny little ‘bump’ (most pax would think this is a very good landing).
- Pilots are trained that, ideally, and at an airspeed of approximately VREF, the main landing gear will touch down simultaneously with the thrust levers reach idle.
- The FCTM stated that “If the flare is too abrupt and thrust is excessive near touchdown, the airplane tends to float in ground effect. Do not allow the airplane to float or attempt to hold it off. Fly the airplane onto the runway at the desired touchdown point and at the desired airspeed.”
- According to the FCOM ̶ Go-Around and Missed Approach Procedure (Appendix B to this Report), a normal go-around is initiated by the pilot flying pushing the TO/GA switch. The pilot flying then calls for ‘flaps 20’, and the pilot monitoring positions the flap selector lever to ‘20’. Both pilots then verify rotation to go-around attitude and that engine thrust increases. The pilot monitoring will verify that the thrust is sufficient for go-around and will adjust as necessary. After a positive rate of climb is verified on the altimeter, the pilot monitoring calls ‘positive climb’. The pilot flying will verify the positive rate of climb on the altimeter and call for ‘gear-up’. The pilot monitoring selects the gear-up and confirms that the flight directors are ‘on’. The pilot flying will limit the bank angle to 15 degrees if the airspeed is below the minimum maneuvering speed. When the aircraft is above 400 ft radio altitude, the pilot flying selects or verifies a roll mode. The pilot monitoring will verify that the missed approach altitude is set, after which further procedures follow for the climb and navigation until the after takeoff checklist is completed.
This procedure is almost identical to the flow we used to call "To L and Back" in older Gulfstreams. See Lessons, below. Our newer airplanes are so automatic that we sometimes forget to do these steps because they always seem to get done. Until they don't.
- It is stated in the FCOM – Autopilot Flight Director System (AFDS) Procedures that the pilots must always monitor airplane course, vertical path and speed. Additionally, the AFDS procedures state that pilots must verify manually selected or automatic AFDS changes; use the FMA to verify mode changes for autopilot, flight director and autothrottle; and should announce changes on the FMA and verify changes to the thrust mode display when they occur.
- The General Information section of the FCTM states that when the term “Set thrust” or “Verify that thrust is set” is used in various places in the FCTM and FCOM, pilots are required to verify the EPR indication.
I am impressed with how much this crew did right that was cancelled out by the few things they did wrong. It is an object lesson in how much the details count in aviation.
[AAIS, §126.96.36.199] The Commander
- After completing a total of 6,283 flying hours, and satisfying the Operator’s selection process, he began his two-month upgrade training to become a commander on the B777 in March 2015. At the time of the Accident, he had flown 1,173 hours as a commander on the B777 and his total flying time was 7,457 hours.
- The Commander performed 54 go-around maneuvers in the simulator with no adverse comments relating to his handling.
- In March 2015 during his upgrade to commander training, the Commander practiced rejected landings from heights below 50 ft, but before the aircraft had touched down. His most recent bounced landing recovery training, and his last windshear recovery training were carried out in March 2015. The Commander stated that he had never practiced normal goarounds after touchdown with the autothrottle armed and active.
- During two of the Commander’s upgrade training sessions, between March and May of 2015, the evaluator had commented on landing technique related to flare and that the landing was towards the end of the touchdown zone. The Operator stated that provided a pilot lands within the touchdown zone, it is considered safe and satisfactory.
- Flight data monitoring (FDM) data records for the Commander indicated that all landings were performed within the requirements of the Operator and within the runway touchdown zone.
- The Operator’s FDM data recorded that, on 13 April 2016 at OMDB, the Commander as the pilot flying had performed a normal procedural go-around following an approach that became unstable due to wind shift resulting in a rapid speed increase.
- After the Aircraft passed the runway threshold, the Commander stated that his attention was outside the cockpit, focused on the far end of the runway. He stated that his practise, in accordance with his training, was not to look at the cockpit instrumentation after the flare was started. In addition, he stated that during the attempted go-around, he was focused on the Aircraft attitude during the rotation and described his state of mind at that stage of the go-around as being “tunnel visioned”. He could not recall if there was movement of the thrust levers from idle position after initiating the go around when he pushed the left thrust lever TO/GA switch.
- During his interview, the Commander stated that his seat was in the optimum position and he had his right hand on the thrust levers during the entire landing phase including the attempted go-around.
- The Commander started to flare the Aircraft at approximately 40 ft radio altitude, approximately 100 m beyond the threshold. The FCTM recommends initiation of the flare when the main gear is approximately 20 to 30 ft above the runway surface.
- Passing 7 ft radio altitude, the Aircraft floated over the runway and at 2 ft radio altitude, the IAS reached 165 kt. There was a 12 kt airspeed increase in approximately four seconds, during which time the descent rate decreased.
- The flight crew did not observe that the speedbrake lever had partially deployed twice during the six seconds the Aircraft main landing gear had cycled between ‘tilt’ and ‘untilt’.
- The Commander had stated that his right hand remained on the thrust levers during the attempted go-around. After pushing the left TO/GA switch, the Commander did not recognise that there was no tactile feedback of thrust lever movement.
- The flight crew did not observe that the FMA modes did not change and that the flight director was not giving pitch guidance. They were not aware that the A/T mode had remained at ‘IDLE’.
- Contrary to the FCOM ̶ Go-around and Missed Approach Procedure, after ‘flaps 20’, the Commander and the Copilot omitted the steps of engine thrust verification and continued to action the procedure from the ‘positive climb’ item.
- After the A/T had changed from ‘SPEED’ to ‘IDLE’ mode at 25 ft radio altitude, the airspeed decreased by 6 kt and was 153 kt at 10 ft radio altitude. As the Aircraft passed the runway aiming point and approximately 480 m beyond the threshold, the airspeed started increasing and at 2 ft radio altitude, it reached 165 kt IAS with the Aircraft approximately 840 m beyond the threshold. There was a 12 kt airspeed increase over approximately four seconds, during which time the groundspeed decreased by 5 kt, and the descent rate decreased from 432 ft per minute to 80 ft per minute. The airspeed increase was 18 kt above the landing reference speed of 147 kt VREF.
- The Investigation concludes that the 12 kt airspeed increase was due to a horizontal windshear as the wind shifted from a tail wind to a head wind component. The wind shift most likely occurred as the Aircraft was descending below 7 ft radio altitude as this was when the Commander first felt the Aircraft being affected by the environmental conditions of hot air raising from the runway surface and the wind shift. During this period, the Commander first exclaimed “Oops” followed by “Thermals”. The Commander made several inputs on the control column, control wheel and rudder in order to maintain wings level and keep the aircraft aligned with the runway centerline.
- The additional lift created because of the increasing performance of the Aircraft contributed to the prolonged floating of the Aircraft over the runway. Even though the Commander was not aware of the increasing airspeed, he had responded to the increasing performance and in an attempt to land, three times made small pitch attitude corrections to lower the nose of the Aircraft. During this time, the Aircraft pitch angle decreased from 2.6 degrees to an average of 1.2 degrees between 5 ft and just prior to the touchdown. The action taken by the Commander was in line with the recommendation of the FCTM ̶ Landing Flare Profile as it was stated that the touchdown body attitude (pitch angle), should be reduced by 1 degree for each 5 kt above the touchdown speed. For UAE521, the touchdown speed was calculated by the flight crew to be 147 kt (VREF30 + 0).
- With the TO/GA switches inhibited because of the right main gear weight-on-wheels (WOW) [‘ground’ mode], pushing the TO/GA switch had no effect on the A/T mode. As designed, when the Commander pushed the left TO/GA switch, the FMA A/T mode remained at ‘IDLE’. As neither pilot had observed the FMA, they were not aware that the A/T mode had not changed to ‘THR’.
- When the Commander called for ‘flaps 20’, 2.5 seconds after the go-around command, both main landing gear were in ‘ground’ mode, with the Aircraft pitch angle increasing towards 7.4 degrees. Although the flight crew stated that they were not aware that the Aircraft had touched down, the Commander was aware that the Aircraft was close to the runway and therefore he limited the pitch angle in order to avoid a tail strike.
- The Operator’s Go-around and Missed Approach Procedure required both pilots to verify rotation to go-around attitude and that engine thrust was increasing, after the pilot monitoring selected flaps 20. The pilot monitoring was then required to verify that thrust was sufficient for the go-around and adjust as necessary.
- The Copilot called “Positive climb” 1.5 seconds after confirming flaps 20, when the Aircraft vertical speed was approximately 592 ft per minute, and the airspeed was decreasing towards 147 kt IAS. The Copilot’s call was followed by the Commander’s call for ‘gear-up’, four seconds after the Aircraft became airborne. The airspeed continued to decrease and was 145 kt IAS at 58 ft radio altitude with the pitch angle increasing towards 8.4 degrees.
- Less than 12.5 seconds from the time that the TO/GA switch was pushed, the Aircraft had insufficient energy remaining to gain further height. The energy loss was aggravated by the landing gear doors opening.
- The Aircraft loss of airspeed was perceived by the Commander as a windshear effect, which prompted him to call “Windshear TOGA”. The sink rate was increasing towards 500 ft per minute as the Aircraft sank below 67 ft radio altitude with the airspeed decreasing below 130 kt IAS. Soon after, the Commander pushed the TO/GA switch and manually advanced both thrust levers fully forward, as per the Operator’s windshear escape maneuver procedure. Only at this time did the Commander realize that the engines were not producing sufficient thrust.
- Eighteen seconds after the initiation of the go-around the Aircraft impacted runway 12L approximately 2,530 m beyond the runway threshold. The Aircraft was controllable until impact, but the height available was insufficient to prevent impact with the runway.
- The Aircraft manufacturer’s post-Accident Performance Evaluation calculated that, as the Aircraft gained height, at approximately 58 ft radio altitude when the landing gear lever was selected to the ‘up’ position, a successful go-around could have been flown had the thrust levers been advanced to go-around thrust by the A/T, or immediately by manual advancement of the thrust levers, and had the pitch been maintained at go-around pitch. The performance analysis indicated that as the engines accelerated, the Aircraft would lose some height but would clear the runway at a minimum height of 17 ft radio altitude before safely climbing away. This hypothetical recovery scenario was possible, had the UAE521 flight crew been [aware] of the Aircraft state, which was not the case.
- The Investigation concludes that the Commander maintained the stabilized approach criteria established by the Operator during the attempted tailwind landing. However, the landing distance was increased due to the early flare, the updraft created by the thermals rising from the runway surface and flight in ground effect which caused the Aircraft to remain airborne beyond the FCTM recommended touchdown of between 305 m to 610 m. Beyond this point, the Aircraft entered a performance increasing windshear as the wind shifted to a headwind.
- Because the Commander was not aware that the Aircraft had touched down and that the TO/GA switches were inhibited, he relied on the Aircraft automation when he pushed the TO/GA switch based on his training for the initiation of a normal go-around. His perception, as well as that of the Copilot, was that the Aircraft was airborne when he pushed the TO/GA switch. However, neither pilot had monitored the engine thrust and Aircraft performance as required by the Go-Around and Missed Approach Procedure. By the time the loss of airspeed was recognized, the actions taken in executing the windshear escape maneuver were too late to avoid impact with the runway.