In my opinion the primary cause of the ET302 accident lies in the take off roll , the final nose dive could have been caused due to structural failure of the elevator and in an extremely improbable case, the infamous MCAS would have had a role to play.
All aircrafts are loaded with passengers, cargo and fuel and maintained within limits of the centre of gravity so that a safe takeoff, airborne segment and landing is carried out. Based on the weight and balance of the aircraft and the airport/runway and environmental conditions, the flight crew calculate how fast the aircraft needs to roll on the runway (speed) and how much engine thrust needs to be generated in order to accelerate with the given mass and other conditions to allow the wings to generate lift from the air mass flowing over the wings. All these parameters have to keep the aircraft takeoff roll within the runway length and after getting airborne keep clear of the obstacles like hills, buildings etc. around the airport.
As per authentic fine granular data provided by Flightradar24, I have plotted the aircraft indicated airspeed and height above the runway. Please refer to the above snapshot of the takeoff roll.
- Initial acceleration is normal till 90kts
- Passing 90kts, the ADS-B data records a positive rate of climb of 128 feet/minute for 8 seconds and gained a height of about 50 feet.
- After the initial climb, she stopped climbing and flew level for 7 seconds at the same time kept accelerating.
- Again there was an attempt to climb as the aircraft speed increased but not more than 50 feet climb could be achieved.
- At the other end of the runway she was around 180 kts and 80 feet above the runway, which is extremely low and very high on speed as compared to a normal takeoff.
A comparison of the acceleration of the ill-fated ET302 and ET302 of other days show similar acceleration rate.
- The acceleration of ET302 is observed to be normal and the comparison with the flights on other days confirm the same.
- The segment to highlight is from the time she achieved the speed of 90kts and thereafter.
- Passing 90kts, a vertical rate of climb is recored for 8 seconds. This rate is similar to the normal takeoff action when the pilot lifts the nose of the aircraft to get airborne (off the ground/runway).
- The rate of rotation of aircrafts generally is 3 degrees per second from 0-15 degrees. It typically takes 5 seconds to get airborne.
- ET302 may have rotated for close to 5-8 seconds but at a much lower speed of 90-100 kts. The typical takeoff speeds at Addis Ababa would be in the range of 155-165 kts depending on a number of variables as explained in the beginning of the blog.
- The aircraft may have rotated due to a number of reasons:
- Incorrect loading of passenger and cargo. The aircraft balances on ground on the pivotal main landing gear wheels located under the winds, close to the belly. If there is too much weight in the rear of the aircraft as compared to the front, the nose could lift as it does on a see saw.
- On the contrary if the nose is too heavy with forward load or if the flap configuration is incorrect as per the calculated performance parameter, the aircraft may find it difficult to takeoff. Video of aircraft finding it difficult to takeoff.
On 23 Apr 2005 a Turkish B-737 was preparing for takeoff from Stuttgart, Germany. After the crew received take-off clearance and after the airplane had started the take-off run, the aircraft pitched backwards and the tail struck the runway. The pilot in command immediately aborted take-off. The airplane pitched back onto the nose gear, rolled out and came to a standstill on taxiway.During the occurrence a flight attendant was slightly injured and the aircraft severely damaged.
The investigation by the German authorities concluded as following: Contrary to the actual loading of the aircraft, the load and trim sheet showed that take-off weight and centre of gravity were within the permitted tolerance. Because of the remaining passengers on board for the destination Düsseldorf which were seated in the rear part of the aircraft, and the luggage stored in the aft cargo compartment, the centre of gravity was now beyond the permitted tolerance.
5. ET302 could have rotated early due to pilot input based on erroneous speed indication on the cockpit instruments. If the pilot’s speed indication is unreliable due to a technical fault, the actual speed may be much lower than the ones displayed in the cockpit. The pilot rotates solely based on the indications from the cockpit.
6. Loss of Pitch Control on Takeoff was the cause of Emery Worldwide Airlines, Flight 17 McDonnell Douglas DC-8-71F accident California February 16, 2000. The National Transportation Safety Board determined that the probable cause of the accident was a loss of pitch control resulting from the disconnection of the right elevator control tab. The disconnection was caused by the failure to properly secure and inspect the attachment bolt.
ET302 rotated at a very low speed causing the tail portion to hit the runway. Since the aircraft continued to accelerate, unlike the Turkish Airline aircraft where the crew aborted the takeoff, there could have been substantial damage to the tail area where the elevator and stabaliser are located. These flight controls help the aircraft in the up and down pitch movement of the nose of the aircraft .
ET302 continued to accelerate towards the end of the runway achieving a speed of 180 kts which is a safe speed to get airborne and leave the ground. In the process, the aircraft was too low and fortunately the runway is like a table top with steep drop towards both ends.
ET302 continued to accelerate to flap retraction speed and reached the maximum operating speed of close to 330kts indicated before the data transmission ends.
Role of MCAS- MCAS is a software designed for the MAX series to prevent the aircraft nose pitching too high during flight thereby preventing a stall. The stall can lead to a loss of lift and a sharp nose dive. There have been accident of the B-737 stall in Rostov of Fly Dubai and Beirut of Ethiopian Airlines. The MCAS triggers when the flaps are retracted at speeds close to 200-220 kts depending on the aircraft weight and Flap configuration. In the case of ET302, the MCAS would have activated only when the flaps were retracted and if the pitch of the nose was very high or if the high pitch indication was falsely generated by a faulty sensor as was the case in the Lion Air JT610 accident.
There are media reports of part of a stabaliser found with a nose down setting. This is expected in a stall situation where the crew is reacting to a violent pitch up attitude. The Fly Dubai accident in Rostov had similar indications of a 12 seconds burst of nose don trim just before the aircraft nose dived.
In this accident I would focus more on the takeoff roll since the pilot had reported flight control problems soon after takeoff and as per eyewitness reports, there was debris falling and smoke seen from the rear end of the aircraft.
A tail strike always leave deep marks on the runway and if there was structural damage and separation of parts they would have been reported by the air traffic controller as found on the runway after the runway inspection.
ET302 may have impacted the tails due to the extremely low speed at which the rotation is though to have occurred. Thereafter, the crew may have tried to salvate the situation by flying level and increasing speed, as can be seen on the ADS-B data. There was an attempt to raise the nose but something prevented this to happen. This could be the flight control problem that the pilot reported. The crew managed to gain height slowly but there was a force preventing them from doing so since the graph shows increase and decrease of vertical speed but slow climb.
The spikes in green are the vertical climb rate and the gradual altitude increase is depicted in blue. The graphs are not similar to the Lion Air JT610 accident flight data.
In my opinion the primary cause lies in the take off roll of ET302, the final nose dive could have been caused due to structural failure of the elevator and in an extremely improbable case, the MCAS would have had a role to play.
Why is the world finding similarities between Lion Air and Ethiopian Air accidents
The data can be misrepresented if it is not plotted using the same variables.