The public prosecutor in Évreux, France, is investigating the conduct of the captain of Aer Lingus flight EI402. The captain allegedly refused at first to land at a military air base after a hoax bomb threat on 5 November, preferring a civilian airport, the prosecutor says12. Aer Lingus says the captain and crew "acted impeccably and were thoroughly professional"13. The Irish Air Line Pilots' Association (IALPA) supports both pilots and calls their questioning in France "generally standard procedure following incidents of this nature occurring within French airspace"3.

What happened to EI402

EI402 left Dublin for Rome at 06:24 local time3. The hoax email sent to Rome's airport operator that morning named two Rome-bound flights, including one from Dublin listed as "EL402"4. See: Italy: hoax bomb threat on two Rome flights. The French prime minister's office said that because the pilot "did not cooperate as expected with authorities", the prime minister ordered fighter jets to intercept and escort the aircraft52. A Mirage 2000-5 and a Rafale from Saint-Dizier escorted the Airbus A320 to Air Base 105 at Évreux6. On board were 151 passengers and six crew. No explosives were found6.

A passenger told RTÉ Radio that the captain said the threat did not seem credible, but that the French Air Force had asked the crew to land at an air base5. Another passenger told the Irish Examiner that the captain was "very calm"7.

Who is investigating

Prosecutor Rémi Coutin went to the base to clarify the conduct of the Irish pilot, who he said had allegedly refused to obey the order to land at a military airfield25. Coutin described this as a specific offence under the French transport code2. The case has been assigned to the air transport research section at Roissy, to the Air and Space Force at Villacoublay and to the Eure gendarmerie group2. France 3 reported that Coutin requested three separate investigations6.

The captain was to be interviewed on a voluntary basis, without arrest, after which the prosecutor would decide whether to take him into custody28. A French regional prosecutor later said the pilot and co-pilot had been questioned37. On Thursday, the Évreux prosecutor's office said the investigation was at an early stage6.

What Aer Lingus and IALPA say

On Thursday night, Aer Lingus said its pilots "fully co-operated with the French authorities regarding this matter, and the engagement of the pilots with the authorities has now concluded"17. Asked whether the crew had initially declined to divert as instructed, the airline had no further comment1. On Friday it added that the captain and crew acted impeccably "in what were very challenging circumstances"13.

IALPA said both pilots cooperated fully and are represented in France by its sister union, the SNPL3. It said pilots have "clear legal responsibilities and authority in relation to the safe operation of their aircraft", and that for an Irish-registered aircraft these are governed both by Irish law and by the law of the state in whose airspace the aircraft is operating3. IALPA said it is satisfied that the pilots' actions were consistent with their professional obligations and "their overriding responsibility for aviation safety"3. It also noted that "any abnormal or unexpected event increases the workload in the cockpit"5.

The rules: PIC authority and interception

Under the EU's Standardised European Rules of the Air, the pilot-in-command has final authority as to the disposition of the aircraft while in command (SERA.2015)9. EASA publishes these rules as Easy Access Rules10. Interception has its own rule. Under SERA.11015, the pilot-in-command of an intercepted civil aircraft shall immediately follow the instructions given by the intercepting aircraft, notify ATC if possible, call the interceptor or intercept control unit on 121.5 MHz and select transponder code 7700 unless ATC instructs otherwise9.

Ireland's AIP sets out the same duties. It adds that if radio instructions from any source conflict with the interceptor's visual signals, the crew shall request immediate clarification while continuing to comply with the interceptor's instructions11. The signal table also gives the intercepted aircraft a way to say "aerodrome you have designated is inadequate": raising the landing gear while passing over the runway at 1,000 to 2,000 ft above aerodrome level and continuing to circle. The interceptor can then lead the aircraft to an alternate aerodrome or release it11.

Crew workload during a bomb threat

SKYbrary's guidance for controllers notes that a crew receiving a bomb threat assesses the situation and, depending on the phase of flight and position, decides how to continue. High levels of stress are normal even when the crew sounds calm, and non-standard radio communication can be expected12. Thursday's threats affected at least 27 flights, after a similar wave earlier in the week35; see also the hoax bomb threats against flights on 3 November and the Aer Lingus airline profile.