ETHIOPIAN FLIGHT ET302 CRASH - PRELIMINARY REPORT - AOA & STAB PITCH TRIM

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FACTUAL INFORMATION
HISTORY OF FLIGHT
On March 10, 2019, at about 05:44 UTC1, Ethiopian Airlines flight 302, a Boeing 737-8 (MAX), Ethiopian registration ET-AVJ, crashed near Ejere, Ethiopia, shortly after takeoff from Addis Ababa Bole International Airport (HAAB), Ethiopia.  The flight was a regularly scheduled international passenger flight from Addis Ababa to Jomo Kenyatta International Airport (HKJK), Nairobi, Kenya.  There were 157 passengers and crew on board. All were fatally injured, and the Aircraft was destroyed.
The following is based on the preliminary analysis of the DFDR, CVR and ATC communications.  As the investigation continues, revisions and changes may occur before the final report is published.
At 05:37:34, ATC issued take off clearance to ET-302 and to contact radar on 119.7 MHz.
Takeoff roll began from runway 07R at a field elevation of 2333.5 m at approximately 05:38, with a flap setting of 5 degrees and a stabilizer setting of 5.6 units. The takeoff roll appeared normal, including normal values of left and right angle-of-attack (AOA). During takeoff roll, the engines stabilized at about 94% N1, which matched the N1 Reference recorded on the DFDR. From this point for most of the flight, the N1 Reference remained about 94% and the throttles did not move. The N1 target indicated non data pattern 220 seconds before the end of recording. According to the CVR data and the control column forces recorded in DFDR, captain was the pilot flying.
At 05:38:44, shortly after liftoff, the left and right recorded AOA values deviated. Left AOA decreased to 11.1° then increased to 35.7° while value of right AOA indicated 14.94°. Then after, the left AOA value reached 74.5° in ¾ seconds while the right AOA reached a maximum value of 15.3°. At this time, the left stick shaker activated and remained active until near the end of the recording. Also, the airspeed, altitude and flight director pitch bar values from the left side noted deviating from the corresponding right side values. The left side values were lower than the right side values until near the end of the recording.
At 05:38:43 and about 50 ft radio altitude, the flight director roll mode changed to LNAV.
At 05:38:46 and about 200 ft radio altitude, the Master Caution parameter changed state. The First Officer called out Master Caution Anti-Ice on CVR.  Four seconds later, the recorded Left AOA Heat parameter changed state. 
At 05:38:58 and about 400 ft radio altitude, the flight director pitch mode changed to VNAV SPEED and Captain called out “Command” (standard call out for autopilot engagement) and an autopilot warning is recorded.
At 05:39:00, Captain called out “Command”.
At 05:39:01 and about 630 ft radio altitude, a second autopilot warning is recorded.
At 05:39:06, the Captain advised the First-Officer to contact radar and First Officer reported SHALA 2A departure crossing 8400 ft and climbing FL 320.
Between liftoff and 1000 ft above ground level (AGL), the pitch trim position moved between 4.9 and 5.9 units in response to manual electric trim inputs.  At 1000 ft AGL, the pitch trim position was at 5.6 units.
At 05:39:22 and about 1,000 feet the left autopilot (AP) was engaged (it disengaged about 33 seconds later), the flaps were retracted and the pitch trim position decreased to 4.6 units.
Six seconds after the autopilot engagement, there were small amplitude roll oscillations accompanied by lateral acceleration, rudder oscillations and slight heading changes.  These oscillations continued also after the autopilot was disengaged.
At 05:39:29, radar controller identified ET-302 and instructed to climb FL 340 and when able right turns direct to RUDOL and the First-Officer acknowledged.
At 05:39:42, Level Change mode was engaged.  The selected altitude was 32000 ft.  Shortly after the mode change, the selected airspeed was set to 238 kt.
At 05:39:45, Captain requested flaps up and First-Officer acknowledged. One second later, flap handle moved from 5 to 0 degrees and flaps retraction began. 
At 05:39:50, the selected heading started to change from 072 to 197 degrees and at the same time the Captain asked the First-Officer to request to maintain runway heading.
At 05:39:55, Autopilot disengaged,
At 05:39:57, the Captain advised again the First-Officer to request to maintain runway heading and that they are having flight control problems. 
At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly. 
At 05:40:03 Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.
At 05:40:05, the First-Officer reported to ATC that they were unable to maintain SHALA 1A and requested runway heading which was approved by ATC.
At 05:40:06, left and right flap position reached a recorded value of 0.019 degrees which remained until the end of the recording.
The column moved aft and a positive climb was re-established during the automatic AND motion. 
At 05:40:12, approximately three seconds after AND stabilizer motion ends, electric trim (from pilot activated switches on the yoke) in the Aircraft nose up (ANU) direction is recorded on the DFDR and the stabilizer moved in the ANU direction to 2.4 units.  The Aircraft pitch attitude remained about the same as the back pressure on the column increased.
At 05:40:20, approximately five seconds after the end of the ANU stabilizer motion, a second instance of automatic AND stabilizer trim occurred and the stabilizer moved down and reached 0.4 units.
From 05:40:23 to 05:40:31, three Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.
At 05:40:27, the Captain advised the First-Officer to trim up with him. 
At 05:40:28 Manual electric trim in the ANU direction was recorded and the stabilizer reversed moving in the ANU direction and then the trim reached 2.3 units.   
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out.
At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position
At 05:40:44, the Captain called out three times “Pull-up” and the First-Officer acknowledged.
At 05:40:50, the Captain instructed the First Officer to advise ATC that they would like to maintain 14,000 ft and they have flight control problem. 
At 05:40:56, the First-Officer requested ATC to maintain 14,000 ft and reported that they are having flight control problem. ATC approved.
From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the control columns which remained aft of neutral position.  The left indicated airspeed increased from approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was approximately 20-25 kt higher than the left. 
The data indicates that aft force was applied to both columns simultaneously several times throughout the remainder of the recording.
At 05:41:20, the right overspeed clacker was recorded on CVR. It remained active until the end of the recording.
At 05:41:21, the selected altitude was changed from 32000 ft to 14000 ft.
At 05:41:30, the Captain requested the First-Officer to pitch up with him and the First-Officer acknowledged.
At 05:41:32, the left overspeed warning activated and was active intermittently until the end of the recording.
At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working.
At 05:42:10, the Captain asked and the First-Officer requested radar control a vector to return and ATC approved.
At 05:42:30, ATC instructed ET-302 to turn right heading 260 degrees and the First-Officer acknowledged.
At 05:42:43, the selected heading was changed to 262 degrees.
At 05:42:51, the First-Officer mentioned Master Caution Anti-Ice. The Master Caution is recorded on DFDR.
At 05:42:54, both pilots called out “left alpha vane”.
At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough.
At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction.  The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.
At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds.  The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down.  The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.
The left Indicated Airspeed increased, eventually reaching approximately 458 kts and the right Indicated Airspeed reached 500 kts at the end of the recording.  The last recorded pressure altitude was 5,419 ft on the left and 8,399 ft on the right. 

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.

AF447 - Unreliable Speed - by Joelle Barthe, Airbus Engineer

Unreliable Speed

By Joelle Barthe

Flight Operations Engineer

Published on SafetyFirst #5

December 2007

1 Introduction

Unreliable speed is one of the difficul situations that a pilot has to face. Once the failure has been identified, a procedure, based on pitch angles and thrst settings, will assist the pilot in safely flying the aircraft.

But the main difficulty is to rapidly detect an unreliable speed situation. Reaction time is crucial, since the aircraft may stall and overspeed conditions could cause aircraft damage.

The effects of pitot probes obstruction on ground

It intended to make ground and flight crew more sensitive to the consequences of obstructed probes, and to prevent take-off with unriliable speed.

But once airborne, how can the crew handle an unreliable speed situation?

This article is based on A320/A330/A340 design. Cockpit effects, identification and troublshooting, remains similar for wide body aircraft and A380, with some specificities covered in the operational documentation.

2 Effects and consequences in the cockpit

Water, ice, dust, ashes, etc. may pratially or totally block pitot probes and static ports. Equally,tubes misconncected to the Air Data Modules (ADM), plastic covers not removed from probes, insect nest, radome damage, may lead to enrroneous pressure measurements.

The consequences of this erroous pressure information, once used by the ADRs, and/or the standby instruments, are the computation and the display of unreliable speed and/or altitude for all users.

Erroneous speed or altitude indications can be suspected, among others, in the following cases:

- Speed discrepancy (between ADR 1, 2, 3 and standby indication),
- The flutuction of the Indicated Air Speed or of the Pressure Altitude.

- Abnormal correlation between basic flight parameters (IAS, attitude, pitch, thrst, climb rate),

- abnormal AP/FD/ATHR behaviour,

- STALL and OVERSPEED warnings or FLAP RELIEF on ECAM that are in contradiction with ar least one of the indicated airspeeds,

- Inconsistency between radio altitude and pressure altitude,

- Impossibility of extending the landing gear by the normal landing gear system.

Nevertheless, it should be emphasized that identifying an unreliable speed indication is not wlways obvious: no single rule can be givien to conclusively identify all possible erroneous indications and the display of contradictory information may confuse the flight crew. Pilots should therefore be aware of unreliable speed symptoms and consequences.

Depending on the effected probe, i. e. pitot probe os static port, differente indications in the cockpit will become unreliable. Therefore the crew should be aware that some of the usual cues to fly could be unreliable as indicated:

3 Identification and Handling of Unreliable Speed situations

Airbus has developed procedures and guidelines to help crews identify and handle an unreliable speed situation.

The Volume 3 of the FCOM and QRH provide the UNRELIABLE SPEED INDIC / ADR CHECK PROC procedure.

In addition, Airbus has developed training material in the Flight Crew Training Manual ( FCTM, available for A320/A330/A340/A380). The FCTM provides information about the causes and consequences of unreliable ADR computations. It also provides information on how to apply the UNRELIABLE SPEED INDIC / ADR CHECK PROC of the QRH.

An interative trainin tool, the e-Briefing, is also available on https://w3.airbus.com/ in the Flight Operations community, under ther heading "Safety and Operational materials".

4 - Procedures

As soon as a doubt about airspeed indication arises, or a relevant ECAM alert is triggered (relative to ADRs failure or discrepancy for instance), the UNRELIABLE SPEED INDICATION/ADR CHECK PROC procedure should be applied by the crew, following this sequence:

1) If the safe conduct of the flight is affected, APPLY THE MEMORY ITEMS, i. e. fly a pitch with TOGA or CLB thrust,

2) If the safe conduct of the flight is not affected, or once the memory items have been applied, LEVEL OFF, if necessaru, and start TROUBLESHOOTING,

3) If the affected ADR can be identified, fly with the remaining ADR.

4) If the affected ADR cannot be identified or all airspeed indications remain unreliable, FLY WITH PITCH/THRUST REFERENCES.

4.1 Memory Items

If the safe conduct of the flight is affected, the flight crew applies the memory items: theses allow "safe flight conditions" to be rapidly established in all flight phases (take-off, clim, cruise) and aircraft configurations (weight and slats/flaps). The memory items apply more particularly when a failure apprears just after take-off.
Once the target pitch attitude and thrust values have been stabilized at or above minimum safe atltitude, or when the safe conduct of the flight is nor affected, the flight crew enter the 2nd part of the QRH procedure: level off the aircraft and perform troubleshooting.

4.2 Troubleshooting and isolation

The table provided in the QRH gives the pitch (º) and thrust (%N1) to be applied to level off the aircraft according to its weight, altitude and configuration, along with flying technique advices.
In situations where most primary flight data are erroneous, some indications may stil remain correct and should consequentely be used to help the crew stabilize the flight path. This is the case for the Flight Path Vector (FPV), reliable if the static ports are not blocked, and for the GPS altitude displayed on the MCDU, when GPS is installed.

When the flight path is stabilized, the flight crew will start the troubleshooting, keeping in mind that sometimes two or even all three ADRs might provide identical but erroneous data (e.g. due to icing conditions, flight in volcanic ashes, etc).Therefore, do not instinctivelu reject an ADR that is suspected to be affected.

If the troubleshooting procedure enables the crew to identify the affected ADRs, then a normal situation can be. resumed.

But if the affected ADR cannot be identified, or all ADRs are affected, then the flight crew will fly without speed reference, using the pitch and thrust tables.

4.3 Flying using pitch/thrust tables

First, the crew has to switch OFF two ADRs and keep one ADR ON, to keep the Stall Warning Protection.
Then, the crew will [bold] fly the aircraft without speed references, using pitch (º) and thrust (%N1) settings.

To fly the aircraft using pitch and thrust settings, the crew will find in the QRH the tables relative to each phase of flight: Climb, Cruise, Descent and Approach, talking into account the aircraft weight, configuration and altitude. With theses tables, the crew will be able to safety land the aircraft.

5 Back UP Speed Scale (BUSS)

In order to dedrease the crew workload in case of unreliable speed, Airbus has developed the Back-UP Speed Scale (BUSS) that replaces the pitch and thrust table. The BUSS is optional on A320/A330/A340. It is basic on A380, being part of the ADR Monitoring functions.

This indication is based on angle of atack (AOA) sensor information, and is therefore not affected by erroneous pressure measumements.

The BUSS comes with a new ADIUR standar (among other new system standards), where the AOA information is provided through the IRs and nor through the ADRs. This enables selecting all ADRs off without loosing the STALL WARNING PROTECTION.
The AOA information provides a guidance area in place of the speed scale. When the crew selects all ADRs OFF, then:

- The Back-Up Speed Scale replaces the PFD speed scale on both PFDs,

- GPS Altitude replaces the Altitude Scale on both PFDs.

The Back-Up Speed Scale then enables to fly at a safe speed, i. e. above stall speeds, by adjusting thrust and pitch.

The BUSS will be displayed once all ADRs are switched OFF. Therefore, on aircraft that have the BUSS, when the flight crew cannot identify the faulty ADR(s) when performing the troubleshooting, or when all ADRs are affected, the flight crew will switch OFF ADRs, and will fly the green area of the BUSS.

However, if the safe conduct of the flight is affected, the memory items must still be applied before troubleshooting.
As the BUSS is associated to the ADR monitoring funcitions, some unreliable speed situations can be automatically detected (e. g. new ECAM warning "NAV ADR 1+2+3 FAULT"), and some ECAM procedures will lead to the BUSS activation by requesting to switch OFF all ADRs.

6 Conclusion

An unreliable speed situatio may be difficult to identify, due to the multiple scenarios that can lead to it. Therefore, training is a key element: indeed the flight crew's ability to rapid detected the abnormal situation, and to correctely handle it, is cricial.

In case of any doubt, the pilot should apply the pitch/thrust memory items, and then refer to the QRH to safely fly the aircraft, and to positively determine the faulty source(s) before eliminating it (them).

In addition, to further assit the pilot in detecting the failure and safely fly the aircraft, Airbus has developed the BUSS, which provides a safe flying range indication.

Finaly, to reduze the probally of experiencing unreliable speed situations, on-ground actions, such as comprehensive maintenance and through pre-flight exterior inspection, should be stressed.