THERE WAS A REPORT OF MALFUNCTION ON THE AUTOTHROTTLE A COUPLE OF DAYS BEFORE - Sriwijaya Flight SJ182 B737-500

 

There was a report of malfunction on the autothrottle a couple of days before to the technician in the maintenance log, said Nurcayho Utomo.

Autothrottle (A/T) Computer Modification

On 16 November 2000, the FAA issued Airworthiness Directive (AD) number FAA AD 2000-23-34 to replace the existing A/T computer with a new P/N. The FAA AD refers to the Boeing Service Bulletin number SB 737-22A1130 dated 24 September 1998.

The AD was applicable to all Boeing Model 737-300, -400, and -500 series airplanes was effective from 8 January 2001. The AD referred to Boeing Alert Service Bulletin number 737-22A1130 which required replacement of the previous A/T computer with improved A/T computer Boeing P/N 10-62017-30 or 10-62017-31.

The AD addressed the problem of split thrust lever movement caused by irregular A/T operation which results in asymmetric thrust conditions causing the airplane to bank excessively and going into a roll.

On 21 October 1998 Boeing issued a Service Letter (SL) number 737-SL-22-039 and on 30 April 2003 issued SL number 737-SL-22-039-A to advice the operator to incorporate the new A/T computer with P/N 10-62017-30 (Smiths P/N 735SUE9-12, 735SUE10-12 or 755SUE2-4) or 10-62017-31 (756SUE3-4).

The 10-62017-30/31 A/T computer has been modified to address in-service reports of asymmetric thrust events and A/T takeoff setting being oscillatory and overshooting the target N1.

The modification included the CTSM function and a revision to the takeoff N1 set logic. The CTSM function is enabled when the flaps are set at less than 15 degrees.

The function will disengage the A/T when the net thrust difference between the two engines exceeds a limit, more than 2.5 degrees of spoiler deployment is used to control the roll attitude of the aircraft, and the aircraft is not on takeoff or go-around.

The A/T new P/N refers to the A/T computer manufacturer (Smiths) SB number 735SUE-22-1266 and 735SUE-22-1267 which is to be enhanced with the following:

• Thrust Split Monitor

• Improved T/O N1 Set-point Algorithm

• Output ARINC bus label 270, bit 27, changed to show internal disengagement.

• Power lever angle (PLA) sensor validity added to disconnect logic.

• Revised forward stop Built-In Test Equipment (BITE) test to improve reliability

• Added the following in-flight BITE messages:

• Improved torque switch bypass logic for thrust levers moving up from aft stop.

• Corrected several BITE nomenclature discrepancies.

The CTSM code was reviewed by Boeing and GE Aviation with no anomalies identified. The A/T computer manufacturer reviewed their historical documents which demonstrated satisfactory completion of required bench testing at the time of certification. The CTSM passed the operational test that was performed during a flight test. This flight test was historical data obtained during the certification program.

The FAA AD 2000-23-34 was performed when the aircraft was being operated in United States and prior to delivered to Sriwijaya Air on 2012 with the updated modification of the A/T computer with P/N 735SUE10-12 or 755SUE2-4.

The Test of Previously Installed Autothrottle Servo

The A/T servo P/N 111RAA3, was manufactured by Smiths Aerospace (now General Electric/GE). The A/T servo of P/N 111RAA3 with S/N 3480 was previously installed on the PK-CLC aircraft and was removed from the aircraft on 22 December 2020 for A/T problem troubleshooting. The maintenance records showed that the unit was installed on PK-CLC aircraft since 1994.

The A/T servo S/N 3480 was sent to Ontic facility in Chatsworth, California, United States of America on 22 January 2021 for testing. During the conduct of the test, it was revealed that the Ontic test bench was unserviceable and the test was rescheduled.

The A/T servo then was sent to Ontic Cheltenham UK and was received on 12 August 2021. The unit was immediately transferred to a secure store for a scheduled test on 9 December 2021.

On 9 December 2021, the test commenced and was witnessed by KNKT, NTSB, Boeing and AAIB UK.

Pilot Duties and Responsibilities

Sriwijaya Air Company Operation Manual (COM) subchapter 1.4.1 described the responsibility and authority of Pilot in Command as follow:

▪ The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation and security of the aircraft.

▪ In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of part 91 to the extent required to meet that emergency.

▪ The pilot in command must comply with this Company Operations Manual, Company directives, Standard Operating Procedures, and CASR.

The following guidance on crew duties was provided to Sriwijaya’s pilot which was listed in the Sriwijaya Air version of the Boeing 737-300/-500 Flight Crew Operations

Manual (FCOM), “Normal Procedure” page NP 11.2 thru 11.4, dated March 18, 2016.

Crew Duties

The general PF phase of flight responsibilities are:

• taxiing

• flight path and airspeed control

• airplane configuration

• navigation

The general PM phase of flight responsibilities are:

• checklist reading

• communications

• tasks asked for by the PF

• monitoring taxiing, flight path, airspeed, airplane configuration, and navigation

PF and PM duties may change during a flight. For example, the captain could be the PF during taxi but be the PM during takeoff through landing.

The mode control panel is the PF’s responsibility. When flying manually, the PF directs the PM to make the changes on the mode control panel.

The captain is the final authority for all tasks directed and done.

The crew must always monitor:

• airplane course

• vertical path

• speed

When selecting a value on the MCP, verify that the respective value changes on the flight instruments, as applicable.

The crew must verify manually selected or automatic AFDS changes. Use the FMA to verify mode changes for the:

• autopilot

• flight director

• A/T

During LNAV and VNAV operations, verify all changes to the airplane’s:

• course

• vertical path

• thrust

• speed

Announcing changes on the FMA and thrust mode display when they occur is a good CRM practice.

The COM subchapter 8.1.10.6 also required PIC to report and record of mechanical irregularities as follows:

Whenever a pilot finds a defective equipment, the PIC will:

1. Check the Aircraft Maintenance Log to see if the item has been previously reported and properly deferred. If the item has not been previously written up, the PIC will record the pertinent information on the Aircraft Maintenance Log.

2. Check the approved Minimum Equipment List to determine if the defective equipment may be deferred and the conditions that must be met.

3. If the defective equipment is not deferrable, the PIC will not allow the aircraft to take off until mechanical irregularity is corrected or acceptable Dispatch Authorization has been issued.

Policy on the Use of Automation

The COM subchapter 8.3.18 described the policy on the use of automation as follow:

Automatic flight systems are designed to enhance flight safety and efficiency and must be used to their fullest extent.

▪ Company’s policy that the highest level of automation appropriate to the task should be used.

▪ The Flight Crew must not allow automation to detract from the overall management of the flight.

The Sriwijaya Air version of the Boeing 737 CL Flight Crew Training Manual (FCTM), page 1.38 described that during all phases of flight, the use of A/T is recommended when the A/P is engaged.

Normal Procedure on Climb and Pass 10,000 feet

The following guidance on climb and cruise was provided to Sriwijaya’s pilot which is listed in the Sriwijaya Air version of the Boeing 737-300/-500 FCOM, page NP 21.39 dated June 15, 2020:

Upset Recovery Procedure

The Sriwijaya Air version of the Boeing 737 CL FCTM, page 7.33, described upset recovery as follows:

For detailed information regarding the nature of upsets, aerodynamic principles, recommended training and other related information, refer to the Airplane Upset Prevention & Recovery Training Aid (AUPRTA) available through your operator and on the ICAO website.

Historically, an upset has been defined as unintentionally exceeding any one or more of the following conditions:

• pitch attitude greater than 25° nose up

• pitch attitude greater than 10° nose down

• bank angle greater than 45°

• less than the above parameters but flying at an airspeed inappropriate for the conditions.

The latest revision of AUPRTA concludes that an upset condition exists any time that an airplane is deviating from the intended airplane state. The AUPRTA has been updated to emphasize the importance of recognition and avoidance of situations that can lead to airplane upsets and to improve a pilot’s ability to recover control of an airplane that deviates from the intended airplane state. An airplane upset can involve pitch or roll angle deviations as well as inappropriate airspeeds for the conditions.

With the focus on upset recognition and avoidance, pilots should understand how to operate the airplane throughout the entire operational flight envelope. Pilots should have practical knowledge of and demonstrate proficiency in airplane performance and handling characteristics.

Upset prevention and recovery training should emphasize the entire operational flight envelope to develop pilot awareness and handling skills in both manual and automated flight.

 Nose Low, High Bank Angles

The nose low, high angle of bank upset requires prompt action by the pilot as altitude is rapidly being exchanged for airspeed. Even if the airplane is at a high enough altitude that ground impact is not an immediate concern, airspeed can rapidly increase beyond airplane design limits. Simultaneous application of roll and adjustment of thrust may be necessary. It may be necessary to apply nose[1]down elevator to limit the amount of lift, which will be acting toward the ground if the bank angle exceeds 90°. This also reduces wing angle of attack to improve roll capability. Full aileron and spoiler input should be used if necessary to smoothly establish a recovery roll rate toward the nearest horizon. It is important to not increase g force or use nose-up elevator or stabilizer until approaching wings level. The pilot should also extend the speed brakes as needed.

High Bank Angles

If the airplane is not in “zero-angle-of-bank” flight, lift created by the wings is not being fully applied against gravity, and more than 1 g is required for level flight.

At bank angles greater than 67°, level flight cannot be maintained within AFM load factor limits. In high bank angle increasing airspeed situations, the primary objective is to maneuver the lift of the airplane to directly oppose the force of gravity by rolling in the shortest direction to wings level. Applying nose-up elevator at bank angles above 60° causes no appreciable change in pitch attitude and may exceed normal structure load limits as well as the wing angle of attack for stall. The closer the lift vector is to vertical (wings level), the more effective the applied g is in recovering the airplane.

A smooth application of up to full lateral control should provide enough roll control power to establish a very positive recovery roll rate. If full roll control application is not satisfactory, it may even be necessary to apply some rudder in the direction of the desired roll.

Only a small amount of rudder is needed. Too much rudder applied too quickly or held too long may result in loss of lateral and directional control or structural failure.

The Sriwijaya Air version of the Boeing 737 Quick Reference Handbook (QRH), page

MAN.1.7, described the upset recovery procedure as follows:

Historically, an upset has been defined as unintentionally exceeding any one or more of the following conditions:

• pitch attitude greater than 25° nose up

• pitch attitude greater than 10° nose down

• bank angle greater than 45°

• less than the above parameters but flying at an airspeed inappropriate for the conditions.

An upset condition is now considered any time an airplane is diverting from the intended airplane state. An airplane upset can involve pitch or roll angle deviations as well as inappropriate airspeeds for the conditions.

The following actions represent a logical progression for recovering the airplane.

The sequence of actions is for guidance only and represents a series of options to be considered and used dependent on the situation. Not all actions may be needed once recovery is under way. If needed, use minimal pitch trim during initial recovery. Consider careful use of rudder to aid roll control only if roll control is ineffective and the airplane is not stalled.

These actions assume that the airplane is not stalled. A stall condition can exist at any attitude and can be recognized by one or more of the following:

• Stick shaker

• Buffet that can be heavy at times

• Lack of pitch authority

• Lack of roll control

• Inability to stop a descent.

If the airplane is stalled, first recover from the stall by applying and maintaining nose down elevator until stall recovery is complete and stick shaker stops.

 

STALL Is Only An Angle Of Attack Problem, It Is Not Directly A Speed Problem

 

 

 

Interpretação dos gráficos acima

No gráfico do meio à esquerda da imagem, logo que o fluxo de ar passando por cima da superfície da asa começa a se separar (curva VERMELHA) inicia-se a dimunuição da razão do Coeficiente Máximo de Sustentação (Cl_MAX) da superfície aerodinâmica. Quando o fluxo de ar se separa totalmente, a curva atinge seu ponto máximo, o qual corresponde ao AoA de Estol e ao Coeficiente Máximo de Sustentação.

Se neste exato momento os SLATS se estenderem (curva MAGENTA, vértice mais alto), observe que o ganho na segurança do voo é substancial, pois tanto o AoA de Estol quanto o Coeficiente Máximo de Sustentação aumentam significativamente.

Supondo-se que neste instante, o piloto presuma que se ele estender os FLAPS (curva VERDE), a aeronave ganharia mais segurança na situação, o gráfico demonstra exatamente ao contrário. Aqui há um engano que tem sido compartilhado pela maioria dos pilotos voando aeronave em altas altitudes, pois observe que imediatamente o AoA de Estol diminui assustadoramente (eixo dos X), e embora o Coeficiente Máximo de Sustentação diminua também, ele ainda permanece maior do que o valor para a separação total do fluxo acima da asa.

Se nesta hipotética situação, as asas ficarem contaminadas com GELO (curva ÂMBAR), o pior acontecerá, pois tanto o AoA de Estol diminui quanto o Coeficiente Máximo de Sustentação.

Se os SPEED BRAKES forem abertos nesse instante (curva MARROM ESCURO), o AoA de Estol e o Coeficiente Máximo de Sustentação farão a segurança do voo ficar mais comprometida.

No gráfico da direita, na imagem, fica bem claro que em altas altitudes, quanto menos veloz  o avião estiver voando, o Ângulo de Ataque para o Estol (AoA_STALL) e o Coeficiente de Sustentação Máximo da asa, serão maiores, pois  o Número Mach estará baixo, mas o que  mais é observado, é comandante de avião à jato voando em altas altitudes tentando aumentar o número MACH do voo através do FMS.

Este piloto não está se importando com segurança do voo, pois existe a possibilidade da aeronave entrar numa emergência, e o Coeficiente Máximo de Sustentação das asas bem como o AoA_STALL ficarem muitíssimo reduzidos quando voando nessa condição. Veja no gráfico da direita na imagem. Observe que quando o Número MACH está muito ALTO, tanto o AoA_STALL quanto o Cl_MAX estão muito reduzidos.

PITCH UP EFFECT

The shape of the wing will also determine the STALL characteristics

 You have to remember for a given MACH number a wing stalls at a given angle of attack when the MACH number increases the value of angle of attack stall decreases.

Você tem que se lembrar que para um dado número MACH uma asa estola em um dado ângulo de ataque, quando o número MACH aumenta, o valor de AoA  de estol diminui.

 

 

Fundamental to understanding angle of attack and stalls is the realization that an airplane wing can be stalled at any airspeed and any altitude. Moreover, attitude has no relationship to the aerodynamic stall. Even if the airplane is in descent with what looks like ample airspeed, the surface can be stalled. If the angle of attack is greater than the stall angle, the surface will stall.

 

Most pilots are experienced in simulator or even airplane exercises that involve approach to stall. This is a dramatically different condition than a recovery from an actual stall because the technique is not the same. The present approach to stall technique being taught for testing is focused on “powering” out of the non-stalled condition with emphasis on minimum loss of altitude. At high altitude this technique may be totally inadequate due to the lack of excess thrust. It is impossible to recover from a stalled condition without reducing the angle of attack and that will certainly mean a loss of altitude, regardless of how close the airplane is to the ground. Although the thrust vector may supplement the recovery it is not the primary control. At stall angles of attack, the drag is very high and thrust available may be marginal. Also, if the engine(s) are at idle, the acceleration could be very slow, thus extending the recovery. At high altitudes, where the available thrust will be reduced, it is even less of a benefit to the pilot. The elevator is the primary control to recover from a stalled condition, because without reducing the angle of attack, the airplane will remain in a stalled condition until ground impact, regardless of the altitude at which it started.

 

Effective stall recovery requires a deliberate and smooth reduction in wing angle of attack. The elevator is the primary pitch control in all flight conditions, not thrust.

   Fundamental para a compreensão de ângulo de ataque e estois (perda de sustentação) é a compreensão de que uma asa de avião pode ser estolada em qualquer velocidade e altitude. Além disso, atitude não tem relação com o estol aerodinâmico. Mesmo se o avião estiver em descida com o que se parece uma velocidade ampla, a superfície pode ser estolada. Se o ângulo de ataque for maior que o ângulo de estol, a superfície irá estolar.

   A maioria dos pilotos são experientes em simulador ou mesmo exercícios no avião que envolvem a aproximação do estol. Esta é uma condição dramaticamente diferente do que uma recuperação de um estol real, porque a técnica não é a mesma.  A presente abordagem para a técnica de estol sendo ensinada para teste está focada em "potenciação" fora a condição de não-estolado com ênfase na perda mínima de altitude. Em altitude elevada, esta técnica pode ser totalmente inadequada devido à falta de excesso de potência. É impossível se recuperar de uma condição estolado sem reduzir o ângulo de ataque e isso certamente significará uma perda de altitude, independente de quão perto o avião está do solo.

  Embora o vetor potência possa completar a recuperação ele não é o controle principal. Em ângulos de ataque de estol, o arrasto é muito alto e a potência disponível pode ser marginal. Além disso, se os motores estiverem em idle, a aceleração poderia ser muito lenta, assim, prolongando a recuperação. Em altas altitudes, onde a potência disponível será reduzida, ela é mesmo menos que uma vantagem para o piloto. O elevador é o controle principal para se recuperar de uma condição de estol, porque sem reduzir o ângulo de ataque, o avião permanecerá em uma condição estolada até o impacto com o solo, independente da altitude na qual ele  iniciou.

    Recuperação efetiva de estol exige uma redução deliberada e suave no ângulo de ataque da asa. O elevador é o controle principal do pitch  em todas as condições de voo, e não a potência.

 

Pilot Tips

Dicas de Piloto
1 - The amber bands limits do not provide an indication of sufficient thrust to maintain the current and airspeed.

1 - As faixas âmbar de limites não fornecem uma indicação de potência suficiente para manter a potência atual e a velocidade aerodinâmica.
2 - The amber bands does not give any indication of thrust limits.

2 - As faixas âmbar não dão qualquer indicação de limites de potência.
3 - The minimum maneuver speed indication does not guarantee the ability to maintain level flight at that speed

3 - A indicação de velocidade  mínima de manobra não garante a capacidade para manter o vôo nivelado nessa velocidade
4 - Flying near maximum altitude will result in reduced bank angle capability; therefore, autopilot or crew inputs must be kept below buffet thresholds.

4 - Voando perto da altitude máxima resultará em capacidade de ângulo de inclinação lateral reduzido; Portanto, as entradas de dados do piloto automático ou da tripulação devem ser mantidas abaixo dos limiares de buffet.  [Agitação aerodinâmica de uma estrutura de aeronave por fluxos separados de camadas de ar em torno das superfícies]

 5 - The use of LNAV will ensure bank angle is limited to respect buffet and thrust margins. The use of other automation modes, or hand flying, may cause a bank angles that result in buffeting.

5 - O uso de LNAV assegurará que o ângulo de inclinação lateral está limitado a respeitar a agitação (vibração que precede o estol) da estrutura da aeronave e às margens de potência. O uso de outros modos de automação, ou voando manualmente, pode causar ângulos de inclinação lateral que resultam em  agitação aerodinâmica da estrutura da aeronave.
 

AirAsia QZ8501 - Elevators

Loss Of Airplane Control In Flight

Boeing's Flight Instructions for Upset Recovery, click here for downloading PDF file

Video clip showing Upset RecoveryTraining on Simulator, click here for downloading WMV file

 

 



Pilot Tip

For airplanes with real-time bank angle protection, the bank angle limiting function is only available when in LNAV    - as in     HDG SEL bank angle protection is lost.

 



The four conditions that generally describe an airplane upset (figure 1) are unintentional:
• Pitch attitude more than 25 degrees nose up.
• Pitch attitude more than 10 degrees nose down.
• Bank angle more than 45 degrees.
• Flight within these parameters at airspeeds inappropriate for the conditions.

STALL RECOVERY.
In all upset situations, it is necessary to recover from a stall before applying any other recovery actions. To recover from the stall, angle of attack must be reduced below the stalling angle. Nose-down pitch control must be applied and maintained until the wings are unstalled. Under certain conditions, on airplanes with underwing-mounted engines, it may be necessary to reduce some thrust in order to prevent the angle of attack from continuing to increase. Once unstalled, upset recovery actions may be taken and thrust reapplied as needed.

In reality example:
During recovery from the upset, pilot rudder and sidestick control inputs resulted in aircraft sideslip and g loadings. These contributed to the displacement of occupants and objects in the cabin, as well as placing lateral accelerations and aerodynamic loads on the vertical stabilizer structure to beyond certified limits.
 During the 18-second duration of the event, vertical accelerations reached peak values of +1.57g and -0.77g. Lateral accelerations reached peak values of +0.49g (right) and 0.46g (left) during four oscillations. Some actions to rectify the upset were similar to those that contributed to damage to the vertical stabilizer attachment fittings on flight AA587 in 2001. The Airbus A300 in that event crashed after separation of the vertical stabilizer.

The TSB further notes that annual recurrent A319/A320 pilot training at Air Canada did not consistently include reference to the hazards of pilot rudder pedal reversals during upset recovery at high airspeeds. This increased the likelihood that pilots would make inappropriate rudder pedal inputs during upset recoveries.

Dica de Piloto

Para aviões com proteção de ângulo de inclinação de asas em tempo real, a função de limitação do ângulo de inclinação só está disponível quando em LNAV – quando em HDG SEL a proteção do ângulo de inclinação das asas está perdida.

As quatro condições que geralmente descrevem um avião em atitude descontrolada (Figura 1) são não-intencionais:

• atitude de inclinação do nariz para cima superior a 25 graus

• Atitude de inclinação do nariz para baixo maior que 10 graus

• Ângulo di inclinação das asas maior que 45 graus. 

• Vôo dentro destes parâmetros em velocidades inadequadas para as condições.

RECUPERAÇÃO DE ESTOL

Em todas as situações descontroladas, é necessário recuperar de um estol antes de aplicar quaisquer outras ações de recuperação. Para recuperar do estol, o ângulo de ataque [das asas] deve ser reduzido abaixo do ângulo de estol [= baixar o nariz]. O controle de inclinação do nariz para baixo deve ser aplicado e mantido até que as asas já estejam com sustenção [aerodinâmica]. Sob certas condições, em aviões com motores montados sob as asas, pode ser necessário reduzir certa quantidade de potência a fim de impedir que o ângulo de ataque continue a aumentar. Uma vez recuperado do estol, as ações para recuperação do descontrole podem ser tomadas e a potência reaplicada conforme necessária.

No exemplo de realidade:

Durante a recuperação da tombada [lateral], as entradas de controle feitas pelo piloto no leme e sidestick resultaram em deslizamento lateral da aeronave e cargas G [= forças gravitacionais]. Estas contribuíram para o deslocamento dos ocupantes e objetos na cabine, bem como a colocação de acelerações transversais e cargas aerodinâmicas sobre a estrutura do estabilizador vertical para além dos limites de certificados. 

Durante os 18 segundos de duração do evento, acelerações verticais atingiram valores de pico de + 1.57 G e - 0,77 G. Acelerações transversais atingiram valores de pico de + 0,49 G (à direita) e 0,46 G (à esquerda) durante quatro oscilações. Algumas ações para retificar a virada foram similares àquelas que contribuíram para danificar os elementos de fixação do estabilizador vertical no voo AA587 em 2001. O Airbus A300 naquele evento caiu após a separação do estabilizador vertical.

A TSB adicionalmente observa que piloto de A319/A320 em treinamento anual  recorrente na Air Canada não nclui consistentemente referência aos perigos de inversões de pedal  do piloto de  leme durante a recuperação de descontrole em altas velocidades. Isto aumentou a probabilidade de que os pilotos fariam movimentos inapropriados do pedal de leme durante recuperações de ‘tombamento lateral descontrolado’ da aeronave

STALL Update Procedure - Aerodynamic Review

Scrap from 17th Performance and Operations Conference in Dubai

Aerodynamic Review

STALL

•  A loss of speed can result in an aircraft reaching the stall AoA
•  BUT it remains an AoA issue

Stall is only an AoA problem [AoA = Angle of Attack]

• Low speed is a common contributing factor

AoA Control –Pitch control effect

The pitch control is a direct AoA command

• The elevators control DIRECTLY the AoA.
• A nose down command has an IMMEDIATE effect:

AoA Control –Thrust effect

• Aircraft with engines below the aircraft Center of Gravity
⇒ Thrust has a significant pitch effect

Stall Recovery

When Aircraft is stalled

•  FIRST: AoA MUST BE REDUCED
•  Release back pressure on stick or column
•  Nose down pitch input may be needed

 Note: Increasing thrust has an adverse effect on AoA reduction for Aircraft with engines below aircraft CG

•  SECOND: If speed needs to be recovered
•  When stall indications cease, increase thrust with care due to possible pitch up effect

  AoA comes first, speed  second

The change from approach to stall versus actual stall is not easy to determine, even for a specialist.

Approach to Stall versus Stall recovery procedure

• The classic recovery procedure associated with “Approach to stall” was characterized by a recovery focusing on:
-  Maximum thrust application
-  Minimum loss of altitude

• Whereas the recovery procedure from an “Actual stall” has always focused on:
-  AoA reduction as first action, followed by a speed recovery if needed

 Need for procedure change

• Drawbacks associated with this dual recovery approach are:
- TOGA application may lead to an increaseof the AoA
- Reluctance to apply nose down input
- Challenge to identify the change from “Approach to stall” to “Actual stall”

• Numerous accidents where the “Approach to Stall” recovery procedure
- was applied whereas the aircraft was actually stalled
- or generated a stall through thrust effect

A single procedure focusing on AoA reduction,           

• as a first action, required to cover both
• the “Approach to Stall” and the “Actual Stall” recovery:

It is called the “Stall Recovery” procedure

New Procedure

The FAA Stall Recovery Working Group issued a generic “Stall Recovery” procedure

•  A generic procedure for ALL types of aircraft
•  One single procedure to cover ALL stall conditions
•  Prevent full thrust/TOGA from being first action
•  Focus on AoA reduction

 Generic Stall Recovery Procedure

STALL WARNING OR AERODYNAMIC STALL RECOVERY PROCEDURE

Immediately do the following at the first indication of stall (buffet, stick shaker, stick pusher, or aural or visual indication) during any flight phases except at lift off.

1. Autopilot and autothrottle…………………………….…… Disconnect

Rationale: While maintaining the attitude of the aircraft, disconnect the autopilot and autothrottle. Ensure the pitch attitude does not change adversely when disconnecting the autopilot. This may be very important in mis-trim situations. Manual control is essential to recovery in all situations. Leaving one or the other connected may result in in-advertent changes or adjustments that may not be easily recognized or appropriate, especially during high workload situations.

2.
a) Nose down pitch control…Apply until out of stall (no longer have stall indications)

b) Nose down pitch trim…….………………………………………..As needed

Rationale:

a) The priority is reducing the angle of attack. There have been numerous situations where flight crews did not prioritize this and instead prioritized power and maintaining altitude. This will also address autopilot induced full back trim.

b) If the control column does not provide the needed response, stabilizer trim may be necessary. However, excessive use of trim can aggravate the condition, or may result in loss of control or in high structural loads.

3. Bank………………………….…………………………………….….Wings Level

Rationale: This orientates the lift vector for recovery.

4. Thrust ……………………………………………………………….. As Needed

Rationale: During a stall recovery, many times maximum power is not needed. When stalling, the thrust can be at idle or at high thrust, typically at high altitude. Therefore, the thrust is to be adjusted accordingly during the recovery. For engines installed below the wing, applying maximum thrust can create a strong nose up pitching moment, if speed is low. For aircraft with engines mounted above the wings, thrust application creates a helpful pitch down tendency. For propeller driven aircraft, thrust application energizes the air flow around the wing, assisting in stall recovery.

5. Speed Brakes…………………….………………………………………. Retract

Rationale: This will improve lift and stall margin.

6. Return to the desired flight path.

Rationale: Apply gentle action for recovery to avoid secondary stalls then return to desired flight path.

Conclusion

• Working together with other aircraft manufacturers, we have:
-  Agreed the principle with the FAA Stall Recovery Working Group
- Issued a harmonized procedure focusing on AoA
   reduction as a first action

When Stalls Most Often Occur
 
Circumstantial evidence shows that most full or near-full stalls of transport aircraft occur in one of five situations, as for other paths to loss of control, often but not always when the aircraft is either in IMC or during ‘dark night’ conditions clear of cloud so that no natural horizon is available:

• During inappropriate response to an un-commanded autopilot disconnect at high altitudes. (Uncommanded AP Disconnect due to malfunction of other systems)
• at low altitudes when the indicated airspeed is unintentionally allowed to deviate significantly from the intended and necessary target (Airspeed Awareness)
• at low altitudes in the presence of frozen deposits on the wings (Airframe Icing)
• during a mishandled go around (Aircraft management and Flying Skills)
• because of insufficient understanding of automation as it affects flight envelope protection systems.
• improper slats/flaps configuration (Aircraft Configuration)

Uncommanded AP Disconnect due to malfunction of other systems is not only liable to create a significant ‘startle factor’ for both pilots but is also likely to remove some of the high level flight envelope protections commonly provided by Fly-By-Wire (FRW) flight control systems. Flying manually at high altitude is not a feature of normal operations and there is not always sufficient awareness of the different ‘feel’ of the flight controls in the high altitude case compared to the routinely- experienced low altitude case. The simultaneous removal of some or all automated flight envelope protections at the same time, often because of an automatic reversion to a lower FBW Control Law, creates a heightened imperative to retain control within that envelope in a situation where a full understanding of the different degree of protection provided by Control Laws, other than "Normal", may not be fully understood or appropriately recalled.