On 5 September 2015, a Boeing 737-800 (3C-LLY) being operated by CEIBA Intercontinental on a passenger flight from Dakar to Malabo, Equatorial Guinea with a planned intermediate stop in Cotonou was in the cruise at FL 350 in day VMC when it reported a mid-air collision with an opposite direction aircraft. This aircraft was an HS 125-700 (6V-AIM) being operated by Senegalair on a medevac flight from Ouagadougou to Dakar and cleared to maintain FL 340 so as to pass the opposite direction 737 with 1,000 feet clearance. The 737 continued a reportedly normal flight overflying Cotonou direct to Malabo despite sustaining wingtip damage but there was no further radio contact with the HS 125. It was subsequently observed on radar maintaining FL350 and progressing westwards on the flight planned track before passing over Dakar and continuing offshore on the same track before eventually beginning an apparently uncontrolled descent into the sea. No wreckage was found and the 7 crew and passengers on board were presumed to have been killed.
An Investigation by the BEA Senegal was commenced the same day. A search for wreckage from the 125 was conducted but was not successful. The FDR and 2-hour CVR from the 737 were removed and downloaded by the BEA France between the 26 and 30 October 2015. The FDR data showed that the aircraft had been level at FL 350 throughout its cruise with the AP and A/T engaged and also "confirmed normal function of both the transponder and TCAS". An ATC radar recording was available which showed most of the post-collision flight path of the 125 and the recorded R/T exchanges between ATC and both aircraft were also available.
Both 125 pilots were Algerian nationals with foreign licence validations. The 56 year-old Captain had 7,658 hours total flying experience which included 2,200 hours on type. He had flown a total of 195 hours during the six months prior to the accident. The 35 year-old First Officer had 3,339 hours total flying experience and had flown a total of 136 hours during the six months prior to the accident. The 45 year-old 737 Captain was a Ukrainian national and a second 45 year-old Captain, of Italian nationality and holding an Equatorial Guinea-issued licence was also on board but his status was not documented. Neither was the flying experience of either Captain. The 33 year-old 737 First Officer was an Italian national holding an Equatorial Guinea licence who had 585 hours total flying experience.
It was established that after departing Ouagadougou, the 125 had climbed to FL340 to cruise but had subsequently requested and been cleared to FL380 for weather avoidance by Bamako ACC. On encountering significant turbulence at FL 380, a request to climb to FL400 was approved but when this level was still turbulent, a descent back to FL340 was requested. FL 360 was initially approved and then the crew requested FL340 which they reported reaching and maintaining. As the aircraft approached the Mali/Senegal border at waypoint GATIL after passing ENINO at FL 340 on ATS route UA 601 (see the chart extract below), an approval for a deviation left of track for weather avoidance had been obtained. First contact of the 125 with Dakar Control, in an area without radar or CPDLC coverage, was a request for continued deviation to the left of track for weather avoidance. The initial ATC response was to maintain FL340, report passing the 'TD' (Tambacounda) VOR and squawk 5040. The response from the 125 crew acknowledged the next required report and confirmed the squawk but not the 'maintain FL 340' which required a further request from the controller before it was obtained. Three minutes later, the crew asked for confirmation that the squawk was 5040 and this was confirmed.
An extract from the AIS upper route chart showing the relevant area. [Reproduced from the AIP]
There were no further calls from the 125 but as it entered the area of radar coverage 10 minutes after the time of the collision, it was seen on the flight planned route at FL 350 instead of the assigned FL 340 and with a squawk of 5004 instead of the assigned 5040. It was observed to continue at FL 350 and pass overhead its destination at that level before continuing on the same track out to sea. When the aircraft reached 48 nm beyond Dakar some 55 minutes after the collision had occurred, the radar returns showed it leaving FL 350 and almost immediately turning to the right and then to the left. Descent continued and the aircraft disappeared from radar as it passed FL 126 at 59 nm from Dakar.
The 737 departed Dakar having filed a flight plan to cruise at FL350. It was transferred from TWR to APP and initially cleared to climb only to FL 290 because of conflicting traffic. Once this had been passed, it was re-cleared to FL350 and transferred to Dakar Control which instructed it to maintain FL350 and report at GATIL. Fourteen minutes later the 737 called at GATIL and was instructed to contact Bamako Control. Only then did the crew advise that they had just passed an opposite direction aircraft descending through their level and that they wanted to file a "near miss collision" concerning this traffic which they initially said had "passed very close to them". ATC acknowledged this and immediately called the 125 but there was no response. The 737 crew then said that they suspected that the traffic in question had struck their wing but that "everything is OK and under control". On hearing this, ATC then advised the 737 crew of the 125 traffic heading for Dakar which had been cleared in the opposite direction at FL 340. It was subsequently established that the 737 crew had not only seen the other traffic but had "felt a shock" and had seen afterwards that the right winglet was torn at the top. The aircraft was eventually transferred to Bamako and it subsequently did not land in Cotonou as planned but overflew it and went directly to the Operator's base in Malabo Equatorial Guinea, which required more than three hours flight after the collision. After landing, damage to the upper right side winglet which corresponded with the crew's accounts of the collision was found (see illustration).
Damage to the right winglet of the 737 found after flight. [Reproduced from the Official Report]
A drawing showing the dimensions of the wingtip damage to the 737. [Reproduced from the Official Report]
The assembled evidence indicated the following:
- the 125 had passed the GATIL waypoint eastbound at the about the same time as the 737 had passed the DEMOL waypoint westbound and that the collision has occurred approximately midway between these two positions 8 minutes after the 125 had made its last radio call.
- the collision occurred in RVSM airspace and both of the aircraft involved were RVSM-approved.
- although the TCAS equipment on the 737 was serviceable, it did not produce an RA in respect of the proximity of the 125.
- although the 125 crew had correctly read back their cleared level as F340, they were actually level at FL350 when the collision occurred.
- the airspace in the south east of Senegal was outside ATC radar coverage.
- after the 125 had not flown between 29 November 2014 and 10 July 2015, a minor discrepancy (200 feet) was reported between the altitudes displayed on the two main altimeters.
- This discrepancy was taken by the Investigation to be an indication that "at least one of the two transponders was transmitting different altitudes to those indicated by the altimeter of the pilot in charge of communications".
- Despite this, the aircraft was "returned to service without intervention on its altimetry system" with a request for a further report.
- It was noted that the Senegalair Operations Manual prohibited entry into RVSM airspace if there was a difference of two hundred feet or more between the two primary altimeters.
- Despite this, the next flight - to Robertsfield, Liberia and back overnight on 22/23 July, involved use of RVSM airspace. During the return leg, the Captain of an Arik Air Boeing 737 reported that the 125 had indicated on his TCAS display that it was 1,000 feet clear of the level of his aircraft when it was visible at the same level. It was concluded by the Investigation that this had demonstrated that at that time "at least one of the two transponders had been transmitting different altitudes to those indicated by the altimeter being used by the pilot in charge of communications".
- After the aircraft completed its flying on 23 July 2015, no defect entry was made in the aircraft's Technical Log and "the company engineer signed the Certificate of Release to Service and the operation of the aircraft continued in this condition in RVSM space until 05 September 2015".
- On 31 August 2015, the aircraft was detected by a West African Height Monitoring Site at FL 360 but when it came into radar cover, it was detected at FL 350 "without any revision or authorisation" and began a climb to FL360.
- on the outbound flight from Dakar to Ouagadougou on the day of the accident, the 125 had triggered a Clearance Level Adherence Monitoring (CLAM) alert over eastern Senegal when it was detected level at FL 310 whilst cleared to fly level at FL 330.
The Investigation considered possible explanations for the un-rectified altimetry discrepancy and concluded that the most likely explanation, as foreseen in ICAO PANS-ATM, could be that the selected transponder was faulty, in which case a radar-equipped ATC Unit would have been able to see that the aircraft was not at the assigned altitude and request that the alternate transponder be used to ensure that the safety net provided by TCAS would be effective. Alternatively, but considered less likely, would be if there was an undetected discrepancy between the altitudes displayed on the two main altimeters with the faulty altimeter on the same side as the engaged AP but with the opposite side transponder in use. In that case, the AP would take up the (wrong) altitude but the transponder would correctly transmit this altitude so that again, a radar equipped ATC Unit would be aware. In this case however, TCAS would continue to function pending detection of the error. However, since copies of relevant maintenance documentation had not been available and in particular because neither the flight recorders nor relevant items of wreckage had been recovered, it was impossible to identify a definitive explanation for the malfunction which had been the origin of the collision.
As context for the performance of Senegalair as an aircraft operator, it was noted that:
- Although the Company was based in Dakar, "agency responsible for the aircraft was in South Africa" and it was considered that "this type of organization can create enormous problems if the operator does not have the resources to deal with unserviceability which result in the grounding of the aircraft or operating restrictions".
- In the weeks leading up to the collision, the Company had only one airplane in operation, only one qualified flight crew and only one qualified ground engineer. This situation was considered to have been conducive to an increased risk, respectively, of the continued aircraft use in non-compliant conditions (as in case of non-compliant RVSM operations), exceeding pilot flight time limitations with increased chances of fatigue (as was found to have occurred in a sequence of 5 flights on 31 August 2015) and compromised dispatch airworthiness (as occurred when the only engineer left on 15 August 2015 and approval of their replacements South African-issued Licence was still pending at the time of the collision).
- the Algerian Pilot Licence held by the 125 First Officer had not been validated to permit him to act as pilot on a Senegalese-registered aircraft.
In respect of ATC, it was additionally noted that:
- the quality of VHF radio communications on UA601 between the 'TD' VOR and the GATIL waypoint which included the area where the collision occurred was poor.
- there was nobody acting as operations supervisor in the Dakar ACC control room which was prejudicial to air navigation safety.
- declaration of the applicable sequence of emergency phases following the report of a collision and the loss of radio communications had not been followed by ATC with no INCERFA or ALERFA phases declared. Not until 35 minutes after the 125 had appeared on radar at the wrong level and was already not responding to radio calls over a prolonged period did ATC (Dakar APP) request the preparation of a DETRESFA message. At this point the aircraft was passing overhead the Dakar VOR at FL350 and the DETRESFA phase was not activated (by Dakar Control) for a further 11 minutes as the aircraft disappeared from radar.
- the ANSP ASECNA (L'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar) only informed the ANACIM (the Senegal Civil Aviation and Meteorology Agency) of two serious ATS incidents caused by the 125 involved in the collision under investigation after a "significant delay".
The Probable Cause of the collision was formally recorded as:
"The HS125's non-compliance with its flight level - it was at the FL 350 which was the level assigned to the Boeing 737 even though minutes before the collision, its crew had confirmed to ATC that they were maintaining FL 340. The Captain of the Boeing 737 reported having seen the other aircraft descend into them."
The Investigation also identified two Contributory Factors as follows:
- Altimetry problems on the HS125 may have contributed to the collision.
- Non-compliance with procedures in the Senegalair Operations Manual which prohibited their entering RVSM space if the difference in the reading on the two primary altimeters was more than 200 feet (or more than 75 feet when preparing for the flight).
A total of 11 Safety Recommendations were made as a result of the Investigation as follows:
Four Safety Recommendations were made in the Preliminary Report published on 14 December 2015:
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should improve the quality of HF radio communication in areas where there is no radar coverage.
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should ensure that it strictly enforces the time limits for filing ATS incident reports in accordance with Aeronautics Regulation No. 13 (RAS 13) [which covers the Investigation of Accidents and Incidents].
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should immediately introduce supervisors to oversee Executive and Planning Controllers as recommended in the Dakar ACC Operations Manual.
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should ensure that controllers rigorously apply the procedures for the initiation of an emergency.
Seven more Safety Recommendations were made on completion of the Investigation:
- that the Senegal National Agency for International Civil Aviation and Meteorology (ANACIM) should require that air traffic control ensures an appropriate increased vertical separation (2000 feet for example) around an aircraft as soon as a flight crew casts doubt on its vertical position or as soon as they have a justified doubt about the altitude of their aircraft that the pilot cannot clearly identify and then, as soon as possible, exit RVSM airspace. Aircraft Operators should be required to include in their Operations Manual a procedure for altitude uncertainty. Such a procedure could, inter alia, require the flight crew to make comparisons between various altimeters to remove the doubt, noting that the STANDBY altimeter uses an independent static source.
- that the Senegal National Agency for International Civil Aviation and Meteorology (ANACIM) should sensitise the various services concerned into strict compliance with the checklists related to the issuance of documents; on the absolute rigour which must accompany the application of any proceedings and that as soon as it is informed, the inspection of aircraft likely to present a hazard is made without delay.
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should, in accordance with the triggering procedure for the emergency phases laid down in ICAO Annex 11, include the circumstances which may justify such measures following the first unsuccessful attempt at communication in its procedures.
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should provide the position of operational supervisor in the Dakar ACC control room as soon as possible.
- that l'Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar (ASECNA) should ensure that its deadline for submitting incident reports complies with the applicable Regulatory requirements (a maximum of 48 hours except in exceptional circumstances).
- that CEIBA INTERCONTINENTAL should provide its flight crew with guidelines (elaboration or modification of approved procedures) for decisions which must be taken in the event of certain circumstances.
- that CEIBA and SENEGALAIR should remind their crews of the importance of external visual vigilance alternating with internal monitoring of flight deck indications.
The Final Report (Part 1, Part 2, Part 3) was completed on 1 August 2017 in the French language only and subsequently published.
Editor’s Note: In the absence of a published English Language translation of the Official Report, this summary has been prepared on the basis of an informal translation and no responsibility can be taken for any errors that may have resulted from this.