Ryanair B738 at Eindhoven on May 31st 2013, unstable approach, loss of speed during go-around
Last Update: June 28, 2014 / 17:48:37 GMT/Zulu time
Incident Facts
Date of incident
May 31, 2013
Classification
Incident
Airline
Ryanair
Aircraft Registration
EI-ENL
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
Airport
Eindhoven Airport, Eindhoven
Airport ICAO Code
EHEH
Main conclusions
Eindhoven Tower Control
- Eindhoven Tower Control, when deciding on the runway in use, did not have information available regarding the upper winds and thus did not take into consideration these winds in the IFR traffic pattern. The choice of landing runway resulted in the aircraft drifting on base leg and encountering a tailwind on final that influenced the rate of decent.
Eindhoven Arrival Control
- Eindhoven Arrival Control had no information about the upper winds . The controller did not take into account the influence of the these winds when giving radar guided approaches. This resulted in a line up too short for the final approach.
- Eindhoven Arrival Control did not follow the procedures correctly regarding the following:
–– inform the flight crew beforehand when radar vectors can be expected for the approach;
–– intercepting an ILS glide path should be executed from below in accordance with published procedures;
–– active monitoring of the aircraft flight path during vectoring;
–– transfer of aircraft from Arrival Control to Tower Control without confirmation that the aircraft is established on the ILS or without coordination.
Flight Crew
- The flight crew did not take into account the influence of the upper winds. In combination with the aircraft’s high vertical profile and high speed in relation to the runway distance, a landing in accordance with standard operating procedures became impossible.
- The flight crew did not challenge air traffic control and postponed the decision to make a go-around. It is likely that the crew’s high level of confidence in the very reliable automation in the cockpit contributed to this.
- The flight crew did not have proper guidance procedures to avoid false glide slope capture in relation to the distance to the runway threshold (during an autopilot coupled ILS glide slope approach from above, under instrument meteorological conditions).
- The flight crew initiated the actions for the stall recovery maneuver according to the Boeing FCTM. A second stick shaker warning occurred after the control column was relaxed and the crew again correctly initiated the stall recovery maneuver.
Boeing
- During an autopilot coupled ILS approach the aircraft, flying at an altitude above the normal 3 degree glide slope, followed the fly-up signal after crossing the 9 degrees false glide slope. This resulted in a nose high position of the aircraft causing the stick shaker warning to occur.
- The Boeing 737NG Flight Crew Training Manual did not warn of possible false glide slope capture with a pitch-up upset during an autopilot coupled ILS approach. This resulted in an ‘automation surprise’ for the flight crew.
Ryanair
- The Eindhoven occurrence was initially reported and assessed by the operator as a minor event which did not warrant CVR and FDR retention.
Separate investigation into ILS Findings from the Eindhoven incident revealed characteristics of ILS signals that were not generally known. During the investigation it became clear that the Eindhoven incident was not unique. Other incidents took place with different types of aircraft, operated by different airlines, on approaches to different airports. These findings led the Dutch Safety Board to conclude that unknown ILS signal characteristics pose a significant threat to aviation safety and the Board decided to address this issue separately. The main conclusions of the separate investigation were:
- The ILS Image Type antenna category signal characteristics of false glide paths and corresponding cockpit instrument warnings do not correspond with generally received wisdom and training.
- Signal Reversal sometimes occurs at approximately 6 degree glide path and always at the 9 degree glide path angle. Additionally, cockpit instruments do not present corresponding ILS warnings.
- The area above the certified 3 degree ILS which is the 5.25 degree glide path and onward, is not part of the ILS Flight Inspection programme and therefore not part of the ILS ICAO certified volume of operation. Consequently, aircraft flying above the certified volume of operation are exposed to risks related to ILS Signal Reversal and subsequent unexpected automatic flight system response resulting in severe pitch up.
- Automated systems on board of aircraft assist the aircrew in performing there tasks on board and should never endanger the aircraft, passengers or crew without giving a clear, recognizable warning and ample time for the crew to react.
- Flight crews’ decisions tot execute a go aurond or to challenge Air Traffic Control seem to be postponed too long when flying high above the normal vertical profile during an ILS apporach. There is reason to believe that the high level of very reliable automation in the cockpit contributes to this and that altitude versus distance basic flying skills are insufficiently practiced.
The report of the ILS signal anomaly was issued contemporaneously.
The DSB reported the aircraft departed Palma Mallorca with three pilots in the cockpit, the captain (ATPL, 4,260 hours total, 3,700 hours on type) was pilot monitoring, the first officer (ATPL, 670 hours total, 410 hours on type) was pilot flying, a trainee first officer was occupying the observer's seat but did not perform any flight duties.
The crew received ATIS before entering Dutch Airspace indicating that runway 21 was active and subsequently performed the approach briefings. The crew recognized that there was the possibility of being offered a shorter than the published approach track. The aircraft entered Dutch Airspace at FL070. After contacting Eindhoven arrival, cleared to descend to 3000 feet and descending, the aircraft so far cleared for the standard arrival route via waypoint GEMTI was put on radar vectors to have the aircraft intercept the localizer about 4-5nm before touchdown decreasing the track miles to touch down. 4nm short of GEMTI, at 4200 feet MSL, the aircraft turned onto a heading of 310, the autopilot was engaged in vertical speed mode commanding a rate of descent of 500 fpm. The aircraft was cleared to descend to 2000 feet.
Descending through 3400 feet on the base leg to an about 6nm final the crew began to configure the aircraft, at 3200 feet flaps 5 and an airspeed of 160 knots were selected. The aircraft was subsequently turned onto a heading of 250 further shortening the track miles onto a 4nm final, the crew noticed they were high on the profile and changed the autopilot mode to level change, armed the ILS approach mode, the speedbrakes were extended to recover the vertical profile. Just before extending the flaps to 15 degrees and reducing the speed to 150 KIAS the crew stowed the speed brakes, the gear was selected down, flaps 30 selected and speed reduced to 140 KIAS. The crew then selected flaps 40 and their approach speed of 135 KIAS.
The aircraft was cleared to land and received information the wind was from 330 degrees at 8 knots gusting 16 knots producing a small tail wind component.
Descending through 1300 feet the first officer advised a successful landing was very unlikely and a go-around should be prepared.
The aircraft was descending with a nose attitude of 0.5 degrees nose down at 140 KIAS, when at 1060 feet the glide slope indicator came alive, went fully down and then went fully up again. The aircraft, flown by the autopilot in ILS approach mode, pitched up rapidly and the engines accelerated from 30% to 90% N1. The captain called go-around, the pitch increased further to 24 degrees nose up, the speed decreased and the stick shaker activated.
Almost at the same time the first officer disconnected the autopilot and pressed the TOGA button.
2 seconds later the aircraft reached a nose up attitude of 26.5 degrees, the airspeed fell to 97.5 KIAS, nose down inputs were provided, the aircraft was about 0.65nm from the runway threshold at an altitude of 1267 feet. The first officer continued the stall recovery maneouver, 2 seconds later the stick shaker ceased, 3 seconds later the stick shaker activated again, the captain helped the first officer to reduce the angle of attack and after 3 seconds the second stick shaker activation ceased, the aircraft was at 1429 feet and 0.45nm from the runway threshold at 103 KIAS and increasing.
The crew finished the stall recovery procedure, then retracted the landing gear, reduced the flaps and went around climbing to 2000 feet. The crew advised of their aircraft going around due to a false glideslope capture.
The aircraft positioned for another approach and 10nm final, intercepted the glideslope from below and performed a safe final approach and landing.
The DSB reported that Eindhoven Arrival Controller did not produce a log entry of the go-around in the daily ATC log. Eindhoven Tower did log the go-around, but because no dangerous situation was reported no safety report was written.
The crew notified the airline's duty pilot and asked whether FDR and CVR should be preserved, however, the crew was not aware of the extent of the occurrence and based on their description the duty pilot assessed the occurrence as a minor stick shaker activation event which did not require the preservation of the black boxes.
The aircraft thus continued its next sector on schedule.
The DSB analysed: "The use of runway 21 at Eindhoven Airport resulted in a crosswind at surface level with a slight tailwind component of around 2 knots. This tailwind was within the ATC operational limits to use the runway as landing runway. However, the winds at 2000 and 3000 feet from the North indicated a 30 knots crosswind was present on base leg for runway 21. This crosswind pushed the radar vectored aircraft towards the runway and thus reduced the length of the ground track to the threshold of the runway. Additionally, with the northerly winds at 2000 and 3000 feet, the aircraft encountered a tailwind on final. The tailwind increased the groundspeed on final requiring the crew to increase the rate of descent. The Eindhoven Tower Controller was unaware of the winds at altitude, this information is not presented in the control tower. Based on the factors mentioned in paragraph 2.11.2, runway 21 and the corresponding approach were in use. The effect of the upper winds would have been significantly less if runway 03 had been in use. The effect of the wind on base leg would not have led to a decrease in the ground track to the runway threshold, more likely it would have led to an increase (See paragraph 3.3). On final, the resulting headwind would have given more time and a decreased rate of decent because of a decrease in ground speed."
The DSB analysed that the preceding three aircraft were all flying the published standard approach procedure in full length, hence the approach controller was not aware of necessary wind drift corrections on radar vectors.
The DSB analysed that the aircraft was flying in instrument meteorologic conditions and thus required to be stabilized at 1000 feet AGL. The landing checklist was completed at about 1400 feet, the first officer attempted to intercept the glideslope before 1000 feet, the captain was unsure the aircraft would be able to continue for landing and decided to see whether the glideslope could be intercepted before descending through 1000 feet AGL.
The DSB analysed: "On final approach, if the crew had visibility of the runway or had better performed the crosschecks recommended in the Boeing FCTM, it would have been clear to them that a safe landing was impossible from their position. As the crew did not have ground contact during the approach, the situational awareness about altitude versus distance to the runway was insufficient to realise that a safe landing could not be made and a go around should have been initiated."
The DSB stated: "At 08.48:41 hours glide slope capture occurred at an altitude of 1060 feet at approximately 1 NM from the runway threshold. The crew later reported the glide slope indicator initially went full down and then up again. Analyses shows that the aircraft had flown parallel to the 3 degree glide slope and had intercepted the 9 degree false glide path which has a signal reversal. The aircraft crossed the 9 degree glide path and was now receiving the 9 degree false glide slope signal. The movement of the glide slope indicator in the cockpit gave the flight crew the impression that the glide slope was valid and ‘alive’. Also no warning flag was presented to inform of any danger. Because ILS signals at 9 degrees glide slope are reversed from the normal 3 degree glide path signals, the aircraft received fly-up information rather than fly-down. The pitch started to increase rapidly from 0.5 degrees aircraft nose down (-0.5 ANU) as the aircraft flew through the 9 degrees glide slope beam and the deviation from the beam increased."
The DSB continued: "Prior to the capture, the following indications were available to the crew to build their situational awareness:
- The glide scope scale and pointer showed over 2 dots deviation for the duration of the approach until approximately DME 1.5 NM.
- The deviation was not decreasing as it normally would when approaching the 3 degree glide slope from below.
- The altitude range-distance derived from the DME distance and altimeters was different from normal.
No faults annunciated to indicate that a false glide slope had been captured. The capture of the glide slope signal allowed the crew to continue the approach as this is one of the checklist items. While the aircraft followed the glide slope commands the pitch increased above the normal attitude for an ILS approach, which is limited to only several degrees nose up. As the pitch kept increasing one of the criteria for an upset condition (pitch attitude greater than 25 degrees nose up) was met. As the combination of a false glide slope and the aircraft auto flight systems following the reversed glide slope signal led to an upset, the event can be described as an automation surprise."
The DSB reported that the airline took measures to prevent a repetition including information about the occurrence being distributed to all crew and the development of new standard operating procedures: "Following this incident, the preventive and recovery barriers in Ryanair’s Loss of Control In-flight (LOC-I) Bow Tie have been reassessed and the operator is developing more prescriptive mitigating measures for intercepting the glide slope from above. The revised policy includes a new horizontal landing gate for ILS interception from above; being the earliest of the following: the FAF, 5 DME, 4 NM for VMC operations and 5 NM for IMC operations."
Boeing advised they would incorporate a pitch up warning when flying above the 3 degree glideslope and also include a software change to limit the climb rate when on an ILS glideslope which has shown to eliminate the pitch up when intercepting the mirror reverse 9 degrees glideslope.
The published procedure [green], the vectors [blue] and actual flight track [red]
Aircraft Registration Data
Incident Facts
Date of incident
May 31, 2013
Classification
Incident
Airline
Ryanair
Aircraft Registration
EI-ENL
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
Airport
Eindhoven Airport, Eindhoven
Airport ICAO Code
EHEH
This article is published under license from Avherald.com. © of text by Avherald.com.
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