The only formation of its kind in the world, called the Big Boys, is making a regular appearance over the city skies, rehearsing for the flypast to mark the Air Force Day on October 8. Flying out of the Chandigarh Air Force Station, the Republic Day and the Air Force Day are the only two occasions when they make their appearance.

Forming up for the flypast, however, is not an easy task and requires careful synchronising of aircraft movement right from the tarmac itself.

The first aircraft to get airborne is the IL-76. "The jet wake of the IL-76's four engines is so heavy that no aircraft can fly 80-90 km behind it," an IAF officer here said. This requires the IL-76 to make a circuit after take-off and come overhead the runway again, which takes about five minutes.

While the IL-76 and AN-32s comprising the Big Boys are based locally, the Dorniers are attached here temporarily from a different base.

As the IL-76 is 2 km behind the edge of the runway, it passes a signal to the other four aircraft to begin their take-off run. The two AN-32s, positioned side by side, with one a little behind, get rolling, with the Dorniers following in a similar pattern 10 seconds later.

As soon as they are airborne, they adjust course to position themselves on their respective side of the IL-76 which by now is just a little ahead, thereby avoiding its jet wake. The formation is maintained till the landing approach, when the IL-76 breaks off first.

"One of the major problems while flying the Big Boys is the dissimilar seize and performance of aircraft. The lowest airspeed that an IL-76 can fly comfortably is 300 km per hour, which also happens to be the Dornier's maximum speed, making maintaining the requisite airspeed a demanding task," a pilot commented.

Although the classic distance between two aircraft flying in formation is "one wingspan", this cannot be applied here because the wing span of each aircraft is different. The wing span of the IL-76 it is about 50 meters, while that of the AN-32 is 26 meters. "The distance maintained between the wingtips of aircraft is 30 meters," said a pilot.

All five aircraft have to be at exactly the same flight level. This is ensured visually by the aircrew rather than relying on instruments.

Also manoeuvring the formation is difficult. While the AN-32s and the Dorniers, being smaller aircraft are relatively easier to manoeuvre, the very size and power of the IL-76 makes it slower to respond to the controls. Though the Big Boys have flown in low visibility, they are not permitted to fly through clouds and they have to make way around them without breaking formation.

Unlike as in formation flying by fighter aircraft, which are single seat and have all round visibility, the pilots in the Big Boys who have other aircraft flying on their right, have to switch over from the left hand captain's seat to the right side for ensuring visibility. "This means that the pilot has to get used to flying from the right side seat, the controls at which are a little dissimilar," a flier said.

All Big Boy pilots have to have fully operational status on their respective type of aircraft and be cleared for formation flying. "Though formation flying is basically for showmanship, it also improves the pilots' co-ordination and skill levels," an officer commented.