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The human operator, the least reliable element of an aerodyne, but the most essential...!
Frank Caron (1991)


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26 September 2016

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Human Factors - B737-800 stall during approach


Abbreviations

A/C
A/P
A/T
AGL
AOA
APP
ATC
B737-800
CRM
CVR
EFS
FCC
F/D
FCOM
FCTM
FMA
FMC
FO

Aircraft
Auto Pilot
Auto Throttle
Above Ground Level
Angle Of Attack
Approach
Air Traffic Control
Boeing 737-800
Cockpit Resources Management
Cockpit Voice Recorder
Elevator Feel Shift
Flight Control Computers
Flight Director
Flight Crew Operations Manual
Flight Crew Training Manual
Flight Mode Annunciator
Flight Management Computer
First Officer

GA
GP
GS
ILS
Kts
LOC
Nm
NNC
PFD
PM
QRH
RA
RWY
SMYD
SP
SPD
TRI

Go-Around
Glide Path
Glide Slope
Instrument Landing System
knots
Localizer
Nautical Mile
Non Normal Check-List
Primary Flight Display
Pilot Monitoring
Quick Response Handbook
Radio Altimeter
Runway
Stall Management Yaw Damper
Safety Pilot
Speed
Type Rating Instructor

gggggg


Objectives

The general goal is to have a look on this particular event from human factors point of view as a new approach to the mishap that happen in the industry.

Its single objective is to be educative. It does not intend to blame or even excuse, but providing another perspective.

This specific accident teaches us a lot. Not only regarding how accidents happen and also how the B737NG has been designed and certified…

At least, the dispute between Turkish Airlines and Boeing will probably bring some updates in the stall procedures from the manufacturer

It does not matter to be a pilot or not, flying with the concerned type of aircraft or not, because the focus is only the operator(s) behavior, with a different point of view: the way people act.

 


Summary of the flight
Flight TK1951, operated by a Boeing 737-800, took off from Istanbul on 25 February 2009 to Amsterdam-Schiphol, with 128 passengers, 4 cabin crews and 3 flight crews.

After a normal flight, while executing an ILS approach for RW 18R, the A/T switched to RETARD mode (for flare) and commanded the engine power on APP IDLE thrust.

The speed drop and the pitch increasing were never recognized by the pilots until the stick shaker was activated during final approach. The aircraft stalled at 460 feet and the subsequent stall recovery procedure was not executed properly causing the aircraft to stall and crash at 1.5 km before runway threshold.

There were nine fatalities including the 3 pilots and 1 Flight Attendant. 117 passengers sustained injuries and only 6 were not injured.



Description of the accident

At 8,500 feet during the descent, the aural landing gear configuration warning (not extended) sounded, and subsequently sounded 4 more times until before localizer intercept.

All these alarms were acknowledged by the pilots, since they silenced the aural warning.

Later, TK1951 was radar vectored at 2,000 feet to intercept the localizer 18R at 5.5 Nm. Because the clearance to descent on GS was delayed, the aircraft was fast and high above GS and had to intercept GS from above. Subsequently, this will increase the crew’s workload.

At 1,950 feet, as the aircraft descended to RWY18R, the left-hand RA suddenly changed its reading from 1,950 feet to minus 8 feet. As it is designed for, this information (-8 feet) was transmitted to the A/T system which activated to RETARD (for flare) and subsequently commands the engine to approach IDLE thrust.

The FO was the PF with the right hand A/P engaged and the right hand RA was working properly.

At 1,330 feet, the right hand A/P (using data from the right-hand RA) captured the GS and tracked the ILS. The A/T still command IDLE thrust.

At 1,000 feet, the approach SPD (163 kts) is still above the target SPD (144 kts) and the landing flaps are not set. The approach is not stabilized as required by the Turkish Airlines Procedures.

At 900 feet, the landing flaps are set to 40.

 

At 770 feet the pilots selected the target approach SPD of 144 kts followed by the landing check-list. Because the captain was "coaching" the FO in conducting the before-landing checklist, the pilots are removed from noticing the decreasing speed.

With A/T still on RETARD mode, the A/P was trying to maintain the A/C on the GS, but the decreasing speed was compensated by an increasing pitch in order to maintain sufficient lift. The pilots seem to be unaware of this.

At 460 feet, with the A/T still at IDLE, the SPD is 109 kts and the stick shaker activates. The FO responded immediately.

At 420 feet, The FO pushed the control column and thrust levers forward. But CVR shows that just as the thrust levers had been pushed halfway forward, the captain called he was taking control. The result of this was that the FO selection of thrust was interrupted. As the FO relinquished the thrust levers to hand over control, the still-engaged A/T immediately retarded them again to RETARD mode (for flare).

Directly thereafter, the A/T was disconnected. But for a period of 7 seconds the thrust levers were not moved forwards from the idle position; 9 seconds passed between the activation of the stick-shaker and the movement of the thrust levers to maximum.

At 310 feet, the A/C pitch reached minus 8 deg.

At 250 feet, realizing the aircraft is still stalling, the pilots re-apply full power, but the remaining height above the ground will not be sufficient for recovering.

TK1951 hit the ground at 95 kts with engines at high power in a nose-high attitude. The empennage struck first and the A/C rapidly decelerated, breaking into 3 sections. The wreckage lies at 1.5 km from the RWY 18R threshold.



Weather and ATC

Weather doesn’t seem to be a factor for this daylight accident.
Windshear, wake turbulence or icing have been excluded.

The line-up for final approach of flight TK1951 took place at a distance of between 5 and 8 Nm before the runway threshold without instruction to descend to an altitude lower than 2,000 feet.

This is not in accordance with the Rules and Instructions air traffic control applied by Air Traffic Control the Netherlands, which are based on the International Civil Aviation Organization guidelines.

A turn-in, whereby interception takes place at between 6.2 and 5 Nm, with no instruction to descend to an altitude below 2000 feet is in deviation of the International Civil Aviation Organization guideline specifying that the aircraft must be flying level on its final approach course before the glide slope is intercepted.



Analysis: flight crew

It is a line training flight. The FO is acting as the PF while receiving Line Training by a Line Training Captain under supervision of a SP seated on the jump seat.

Due to the fact that the localizer signal was intercepted at 5.5 Nm from the runway threshold at an altitude of 2,000 feet, the GS had to be intercepted from above.

As a result, the crews were forced to carry out a number of additional procedures, resulting in a greater workload. This also caused the landing checklist to be completed during a later moment in the approach than standard operational procedures prescribe.

Despite the indications in the cockpit, the cockpit crew did not notice the too big decrease in airspeed until the approach to stall warning.

With the cockpit crew - including the SP - working to complete the landing checklist, no one was focusing on the primary task: monitoring the flight path and the airspeed of the aircraft.

It can thus be concluded that the system based around the presence of a SP on board flight TK1951 did not function effectively.



Analysis: systems malfunction

The only fault so far discovered on the aircraft was in the left-hand RA which read minus 8 feet altitude throughout, although the right-hand RA functioned correctly.

The left RA failed twice on approach in the 2 days prior the crash during similar situations commanding the A/T to RETARD mode (for flare).

The inquiry was unable to explain the reason for the malfunction, although it says the radio altimeter started reading incorrectly almost as soon as the aircraft took off from Istanbul.

The problems with radio altimeter systems in the Boeing 737-800 fleet had been affecting several airlines, including Turkish Airlines, for many years and were known to Boeing and the FAA.

Several airlines, including Turkish Airlines, regarded the problems with radio altimeter systems as a technical problem rather than a hazard to flight safety.

As a result, the pilots were not informed of this issue.

Not all certified B737 operating software versions for the A/T and FCC respond to an erroneous radio altitude signal in the same way.

This situation is undesirable, especially in cases where an airline is using several versions that respond differently and without having informed its pilots



Analysis: the radio altimeters

A peculiar systems organization is managing the approach:

  1. The FO is using the right hand A/P, which is engaged and follows the ILS based on the right-hand RA information.
  2. The A/T is engaged
  3. The left-hand RA is the primary provider of information to the A/T.

The final investigation report states that this last feature (left-hand RA is the primary provider of information to the A/T) is a "relic" from the 737's certification in which the original design prioritized provision of information to the captain's position.

The cockpit crew did not have information regarding the interrelationship between the (failure of the) left-hand RA and the operation of the A/T. Of all the available indications and warning signals, only a single indication referred to the incorrect A/T mode, namely the ‘RETARD’ annunciation on the PFD, and the minus 8 feet displayed in the Captain RA window.

With the knowledge available to them at that time, the crew had no way of understanding the actual significance of these indications and warning signals and could not have been expected to determine the pending risk accurately.



Analysis: the Autothrottle

The A/T is working fine during all the approach. Its behavior is just the consequence of the information provided by the left-hand RA.

As a result of intercepting the GS signal from above, it was quite normal that the A/P commands the A/T to reduce to IDLE thrust in order to go down intercepting the GP.

The aircraft did not react as it was above GP, but at it was minus 8 feet (below the ground). As it is design for, the A/T reacts as the aircraft is at minus 8 feet RA and command the engine to IDLE thrust in order to initiate the flare. During normal conditions the A/T begins retarding thrust at approximately 27 feet RA so as to reach idle at touchdown.

Therefore, the above GS situation masked the faulty left-hand RA consequences on the A/T.

That is also the reason it remains idle all the time until it was disconnected after the Captain took over. The same reason the thrust levers came back to idle after le pilots reacted to stall applying (unfortunately not full) power.

Reminder for pilots

  • FLARE will armed during a dual A/P ILS approach, flare (armed in white) will be displayed after LOC and GS capture and below 1,500 feet RA. The second A/P couples with the flight controls and A/P GA mode arms.
  • FLARE (engaged in green) will be engaged, during a dual A/P ILS approach, at 50 feet RA. FLARE accomplishes the autoland flare maneuver.
  • If FLARE is not armed by approximately 350 feet RA, both A/Ps automatically disengage.
  • The A/P flare maneuver starts at approximately 50 feet RA and is completed at touchdown:
  • FLARE (engaged in green) is annunciated and F/D command bars retract.
  • The A/T begins retarding thrust at approximately 27 feet RA so as to reach idle at touchdown. A/T FMA annunciates RETARD.

When the copilot removes hand from thrust levers (only pushed halfway), the thrust levers come back to IDLE thrust. Thrust levers won’t return to full power for the next 7 seconds. This is possible because:

  1. left RA is still blocked to  minus 8 feet
  2. therefore, the A/T system still engaged in RETARD mode (for flare).


The stall

TK1951 entered a stall situation with the autopilot engaged. The autopilot was disengaged at an altitude of between 400 and 450 feet AGL.

Test flight data by Boeing and subsequent analysis of these data have demonstrated that when the aircraft has stalled the altitude loss for recovering from the stall situation after selection of maximum thrust is approximately 500 to 800 feet.

The remaining altitude of 400 to 450 feet in this case, was not sufficient to recover the situation.

The fact that the thrust levers were not immediately moved to their maximum thrust positions in accordance with the approach to stall recovery procedure indicates that the crews were not sufficiently trained to deal with a situation of this type.

The information featured in the QRH regarding the use of the autopilot, the A/T and the need for trimming in the approach to stall recovery procedure is unclear and insufficient.



About turkish airline operations procedures
It is no clear if there is a requirement on calling out FMAs, within Turkish Airlines Pilots. Even though it has been demonstrated that calling out these annunciations raises the pilots' awareness of the automatic flight system status. In accordance with Turkish Airlines standard operating procedures, the approach should have been aborted at 1000 feet followed by a GA, as the approach had not yet been stabilised at this time.

About safety reporting

As a part of its quality assurance programme, Turkish Airlines has drawn up an internal auditing schedule. None of the audits conducted up until the time of the accident yielded any findings as regards adherence to the standard operating procedures described in the Operations Manual or the application of CRM procedures.

In 2008, the Flight Safety Department received 550 aviation safety reports from cockpit crews. None of these reports concerned problems with RA systems, unintended warnings relating to the landing gear, ground proximity warnings or A/T ‘RETARD’ mode indications during approach.

Each year, the Flight Safety Department conducted approximately fifteen incident investigations. No investigation was ever conducted with regard to problems involving RA systems.

This situation is not limited to Turkish Airlines.

Failure to report such problems limits the effectiveness of existing safety programmes. This can result in an inaccurate assessment of risks by both airlines and aircraft manufacturers, limiting their ability to manage risks.



Crew bahavior and CRM

Again and again, it is one of the weakest aspect in this accident.

It is a special flight (training) with a special set of crew: 3 pilots, including a SP fully dedicated to monitoring and supervising the possible operational slips of the training actions of the two pilots manipulating the controls.

Some critical comments can be made:

  1. Captain, FO or SP never express concern about the Landing Gear Configuration warning that sounded 5 times from 8,500 feet up to 2,000 feet,
  2. Captain never notice his RA was displaying minus 8 feet (it is quite difficult for FO or SP to monitor this specific data on Captain PFD),
  3. Captain, FO or SP never stated any RETARD mode on FMA,
  1. Captain, FO or SP never noticed the speed was decreasing (event though the stated “Speed Check” while setting Flaps 40),
  2. Captain, FO or SP never notice the increasing pitch during final approach,
  3. FO never stated the thrust levers were pushed halfway ONLY, during the stall to his Captain taking over,
  4. Captain or SP never noticed the thrust levers were pushed halfway ONLY during the stall,
  5. Captain, FO or SP never noticed the thrust levers were back to IDLE,
  6. Captain, FO or SP never notice the A/T disconnect.

Crew resource management and crew communications during the approach were not in accordance with the standard operating procedure of Turkish Airlines regarding cockpit communication.



Dutch investigators main conclusion
During the accident flight, while executing the approach by means of the instrument landing system with the right A/P, the left RA system showed an incorrect height of -8 feet on the left primary flight display. This incorrect value of -8 fee tresulted in activation of the ‘retard flare’ mode of the A/T, whereby the thrust of both engines was reduced to a minimal value (approach idle) in preparation for the last phase of the landing.

Due to the approach heading and altitude provided to the crew by air traffic control, the localizer signal was intercepted at 5.5 Nm from the RWY threshold with the result that the GS had to be intercepted from above. This obscured the fact that the A/T had entered the retard flare mode. In addition, it increased the crew’s workload.

When the aircraft passed 1000 feet height, the approach was not stabilised so the crew should have initiated a GA. The right A/P (using data from the right RA) followed the GS signal. As the airspeed continued to drop, the aircraft’s pitch attitude kept increasing. The crew failed to recognise the airspeed decay and the pitch increase until the moment the stick shaker was activated. Subsequently the approach to stall recovery procedure was not executed properly, causing the aircraft to stall and crash.”


Dutch investigators recommendations: technology
  1. Boeing should improve the reliability of the radio altimeter system.
  2. The FAA and EASA should ensure that the undesirable response of the A/T and FMC caused by incorrect RA values is evaluated and that the A/T and FMC is improved in accordance with the design specifications.
  1. Boeing, FAA and EASA should assess the use of an auditory low-speed warning signal as a means of warning the crew and - if such a warning signal proves effective - mandate its use.


Dutch investigators recommendations: operational
  1. Boeing should review its ‘Approach to stall’ procedures with regard to the use of A/P and A/T and the need for trimming.
  1. Turkish DGCA, ICAO, FAA and EASA should change their regulations in such a way that airlines and flying training organisations see to it that their recurrent training programmes include practising recovery from stall situations on approach.


Dutch investigators recommendations: Safety Management Systems
  1. FAA, EASA and Turkish DGCA should make (renewed) efforts to make airlines aware of the importance of reporting and ensure that reporting procedures are adhered to.
  2. Boeing should make (renewed) efforts to ensure that all airlines operating Boeing aircraft are aware of the importance of reporting.
  1. Turkish Airlines should ensure that its pilots and maintenance technicians are aware of the importance of reporting.
  2. In light of the deficiencies uncovered in this investigation, Turkish Airlines should adjust its safety programme.


Dutch investigators recommendations: ATC
  1. The ANSP, Air Traffic Control the Netherlands (LVNL) should harmonise its procedures for the lining up of aircraft on approach - as set out in the Rules and instructions air traffic control - with ICAO procedures. LVNL should also ensure that air traffic controllers adhere to these rules.
  1. The Dutch Transport and Water Management Inspectorate (IVW) should monitor LVNL’s compliance with national and international air traffic control procedures.


The dispute: the boeing approach to stall recovery procedure

Turkish Airlines says the A/T "kicked back unexpectedly" and that Boeing "had not mentioned", in its documentation, a need to disconnect the A/T during the (stall) procedure.

The carrier also describes the relationship between the left-hand RA and the A/T as "error-prone", adding that it was not explained in Boeing's documentation for flight crews until after the crash.

While acknowledging that disengagement of the A/T is not described in the recovery procedure, the Dutch Safety Board highlights the problems of incomplete knowledge of the aircraft's interdependent systems. It points to the crew's suffering from "automation surprise" with respect to the A/T behaviour - during both the original loss of thrust on approach and the attempted stall recovery.

Turkish Airlines also claims that simulator tests show that a height of at least 500 feet is required for the 737-800 to recover successfully from a stall, and that the ill-fated jet was already below this level when the pilots initiated the recovery procedure.

In September 2009, Boeing said it was "issuing a reminder” to all 737 operators to carefully monitor primary flight instruments during critical phases of flight".

Up to today, no bulletin has been disseminated yet.

The problem is that regarding stall recovery, there are 2 procedures as it is explained in the FCTM (paragraph Stall Recovery, p.7.8 to 7.11):

  1. Approach to stall recovery.
  2. Recovery from a fully developed stall.

The extract below is from the QRH MAN paragraph.

Boeing Approach to stall recovery procedure

The Approach to stall recovery procedure is explained in in similar ways either in the FCTM (p7.8 to 7.10) or in the QRH MAN 1.1. In each there is no mention the A/T should be disconnected…



The dispute: the boeing recovery from a fully developed stall procedure

The Recovery from a fully developed stall procedure is detailed in FCTM 7.11 only.

In this part, Boeing emphasize: “a recovery from an approach to a stall is not the same as recovery from an actual stall”.

Thus, Boeing splits the two procedures according to their controllable context:

  • The Approach to stall recovery procedure (a stall warning) is a controlled flight maneuver.
  • A (developed) stall, is an out-of-control condition, but recoverable. The Recovery from a fully developed stall (procedure) is dedicated for such a (developed) stall.

After this, Boeing added a note:

Anytime the airplane enters a fully developed stall, the A/P should be disengaged and the A/T should be disconnected.

Therefore, Turkish Airlines and the Dutch Investigation Board are not fully correct when they acknowledge the disengagement of the A/T is not described in the recovery procedure, since they do not really mention which one.

Now, when TK1951 experienced a fully developed stall, the A/T was already disconnected.



The dispute: stall identification as described in FCOM 9.20.10
Stall identification and control is enhanced by three systems that work together to help the pilot identify and prevent further movement into a stall condition:
  1. yaw damper,
  2. Elevator Feel Shift (EFS) module and,
  3. (3) speed trim system. These

During high AOA operations, the SMYD reduces yaw damper commanded rudder movement.
The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure.

The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are:

  • on the ground,
  • radio altitude less than 100 feet
  • and autopilot engaged.

However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.

As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane. With the column aft, the amount of column force increase with the onset of EFS module is more pronounced.


References
  • Boeing 737 Quick Reference Handbook, March 25, 2010
  • Boeing 737 - 600/700/800/900/900ER Flight Crew Training Manual, October 31, 2007
  • Boeing 737-900ER Flight Crew Operations Manual, March 25, 2010.

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