Description

On 22 November 2000, near Birmingham UK, a loss of vertical and lateral separation occurred between a Boeing B757-200 being operated by Britannia Airways on a passenger flight and a formation flight of two F-15Es being operated by the United States Air Force (USAF). The serious loss of separation took place in Class A airspace in Instrument Meteorological Conditions (IMC) and was investigated as a Serious Incident by the Aircraft Accident Investigation Branch (AAIB UK, being judged to have been too dangerous to be examined by the routine AIRPROX review process administered by the UK AIRPROX Board.

It was established that the second aircraft in the formation had not heard, and therefore not actioned, an ATC instruction to climb to FL110 to deconflict with the airways traffic. The separation between the passenger aircraft and the second F15 was estimated to have been less than 116 m horizontally at the same altitude.

Investigation

The following are extracts from the UK AAIB Report of the Investigation:

About one minute after initial contact with MTC [Midland Terminal Control] the B757 was recleared to FL100. The controller acknowledged the B757 crew report on reaching FL100 and advised "MILITARY TRAFFIC IN YOUR ELEVEN O'CLOCK POSITION CROSSING LEFT TO RIGHT, ONE THOUSAND FEET ABOVE". The B757 crew acknowledged the advice and although the aircraft remained in cloud, they immediately began a visual search for the traffic, which their Traffic Alert and Collision Avoidance System (TCAS) was indicating one thousand feet above. The cloud proved to be too thick for visual contact with the military traffic, but the crew remained looking out as the TCAS contact passed clear down their right side.

Shortly after the traffic passed clear and, whilst still in cloud, the commander and the first officer suddenly became aware of an aircraft in their left 'half-past ten' position at very close range and at about the same level. The aircraft, which they were immediately able to identify as a twin-tailed fighter and later as an F15, passed rapidly across the B757's nose and disappeared down their right side. The B757 crew heard the noise of the F15's engines and their aircraft encountered its wake turbulence. There was no time for the B757 crew to take avoiding action. Subsequent analysis of radar data indicated that at the closest point of approach the two aircraft were separated by less than the minimum range detectable by the radar which is 0.0625 of a nautical mile. As far as is known, none of the cabin crew or passengers saw the F15, but the cabin crew felt the disturbance as the B757 flew through the F15's wake. […]

The two F15 were two seat modification ”E” flying on a training mission with a pilot under training on the front seat of the second fighter in formation. According to the report the trainee was in current flying practice on the single seat F15C, but there are several significant differences between the F15C and the F15E, and an F15E instructor pilot was in the rear seat.

The two aircraft took off […] approximately 20 seconds apart and took up a 'trail' formation with the No 2 aircraft about two miles behind the leader. In accordance with standard procedures for this type of formation only the lead aircraft was transmitting a Secondary Surveillance Radar (SSR) code (Squawk). The aircraft climbed through cloud, with the No 2 aircraft maintaining position by use of radar, and levelled at FL100 in Visual Meteorological Conditions (VMC) [Visual Meteorological Conditions]. Part of the briefed flight profile included an aircraft systems check for both aircraft to be carried out in VMC. The procedure for the checks involved a change of lead aircraft. The formation No 2 completed his checks and began to close on the lead aircraft to take the lead position, but the formation entered IMC, and the No 2 aircraft aborted the change of lead and dropped back to about a 1.5 mile trail. In an attempt to regain VMC the lead aircraft requested from ATC a climb to FL110.

Not long after the F15s became airborne, ATC [London Joint Area Organisation Central - LJAO - a Military radar service] cleared the F15s to cross the Daventry Radar Corridor at FL100, and the F15's request to climb to FL110 was made to LJAO shortly after the aircraft entered the Daventry Radar Corridor. The controller's initial response was for the flight to maintain FL100, but she contacted the MTC controller by landline to co-ordinate a climb. The MTC controller agreed the higher level and LJAO later cleared the F15 flight to climb to FL110.

The leader immediately began a climb to FL110, but the No 2 aircraft did not hear the ATC clearance and maintained FL100. The two pilots of the No 2 aircraft later noticed that their radar showed the leader to be above their level, and they began a discussion of the indication. At about this time the front seat occupant was vaguely aware of a 'shadow' flashing rapidly down his right side. Shortly thereafter the LJAO controller asked the flight to confirm that both aircraft were level at FL110, and at this point the No 2 climbed rapidly to FL110. […]

There was some confusion with the pre-notification of the incident F15 flight. A flight plan had been filed for a pair of F15s, call sign EAGLE 31, and a printed flight progress strip showing a flight of two aircraft planning to use the Daventry Radar corridor had been prepared and was at the LJAO controller's position. At 1003 hrs Lakenheath Departures called LJAO to pre-notify F15 traffic for the Daventry Corridor, call sign BOLAR 31. The LJAO controller confirmed that this traffic was in fact the same flight pre-notified as EAGLE 31, and prepared a hand written flight progress strip reflecting the new call sign, but he assumed that the flight was now a single aircraft and he annotated the hand written strip accordingly. The printed strip for EAGLE 31 showing a flight of two aircraft was discarded.

The MTC controller advised the B757 of the crossing military traffic 1,000 feet above, and this was acknowledged by the crew who stated that the traffic was indicating on their TCAS. At this stage MTC had not been advised of the second F15 and neither the controller nor the co-ordinator had noticed the primary radar return of the rear F15 which was partly obscured on the radar display by the lead aircraft SSR label.

The Report makes the following important clarifications of military formation flying procedures used in the UK and valid at the time of the incident:

ATC procedures to be followed by aircraft flying in formation are outlined in CAA Manual of Air Traffic Services Part 1 (MATS Part 1) and MOD Joint Service Publication 318, Part 2 Air Traffic Control General (JSP 318A). MATS Part 1 states that clearance for formation flights to enter controlled airspace may be granted provided the aircraft of the formation can maintain separation from each other visually and all aircraft are able to communicate with the formation leader. MATS Part 1 goes on to state that all ATC instructions and clearances will be addressed to the leader.

JSP 318A states that the formation leader is responsible for separation between the individual units comprising the formation. For separation from other aircraft, formations may be considered as a single unit provided that the formation elements are within one nautical mile both horizontally and longitudinally and are at the same level or altitude. At the controller's discretion, these distances may be increased to 3nm and/or 1,000 feet vertically. [Not in conformance with ICAO SARPs].

While MATS Part 1 covers only formations that can maintain separation from each other visually, JSP 318A outlines procedures to be followed for 'stream' formations in which formation elements may maintain separation from each other either visually or by radar or by the use of station keeping equipment. For stream formations of more than one mile but less than three miles in length only the lead aircraft is required to squawk Mode 3/A and Mode C. Controllers are to identify the full extent of the stream formation during radar handovers, when effecting co-ordination and when passing traffic information. There is no requirement for formation elements not in visual contact with the leader to confirm that they have received or acted upon ATC instructions or clearances.

The Report of the Investigation included the following conclusions:

• […]In particular the MOD [Ministry of Defence] has reduced the maximum permitted separation between aircraft within formations receiving an ATS service and introduced further restrictions for formations flying in category A to E airspace. In addition revised RT [radio telephony] procedures aimed at preventing confusion over the number of aircraft in formation and ensuring that all aircraft are at the formation's assigned level or altitude have been introduced. NATS and MOD have independently but co-operatively commissioned research into SSR garbling and the problems associated with formations and Short Term Conflict Alert (STCA) with the aim of allowing all elements of a formation to be allocated individual SSR codes.
• It was evident during the investigation that there are differences between the ATC formation procedures laid down in MATS Part 1 and those in JSP 318A. It was also noted that some civil controllers were unaware that some formation elements operating in accordance with JSP 318A would not be in visual contact with the leader. Whilst there is no evidence to suggest that these differences in procedure were causal in this incident, they are not conducive to a good understanding between civil and military controllers and they make it difficult for civil controllers to achieve full situational awareness. The CAA has recognised this issue and has requested more information on formation procedures from the MOD with a view to their publication for the benefit of civil air traffic controllers.
• The system of formation call sign allocation by the USAF has the potential to cause uncertainty regarding the actual number of aircraft in a formation. One of the interim amendments to JSP 318A introduced by the UK MOD requires ATC controllers to ensure that information on the number of aircraft in the formation is obtained before providing a service.

The Full UK AAIB Report included two Safety Recommendations:

• Recommendation 2000-71: The CAA and NATS should implement as soon as possible procedures by which the safety assurance based on the use of SSR is established for aircraft operating in formation.
• Recommendation 2001-31: The Ministry of Defence should extend the applicability of its recently revised RT procedures for formation operations to include Class F and G airspace.

Further Reading

LOS

• Loss of Separation,
• AIRPROX,
• Separation Standards;

AGC

• Air-Ground Communication,
• Active Listening in ATC

HF

• Information Processing,
• ATC Expectation Bias.