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.