Airbus A320 Experience

In 2009 The Flying Engineer was given a rare chance, thanks to the crew, to ride the jump seat of a Mandala Airlines Airbus A320 from Jakarta to Surabaya. The article followed the flight step by step and explained the avionics and systems along the way. The author had spent six months working on Airbus flight management systems at a North American avionics maker, and this was the first time he saw them in action.

Before departure

Flight RI 276 was delayed by events that morning. The aircraft was PK-RME, an A320-232, parked at the new Terminal 3 of Soekarno-Hatta airport, opened in April 2009 and home to Indonesia AirAsia (red) and Mandala (blue). The captain was Henry Saut Maruli and the first officer was Firman Joseph.

The load sheet showed a zero fuel weight of 55.6 tonnes with a CG at 31.5 percent of the mean aerodynamic chord, which is tail heavy, so Firman turned the trim wheel by hand. The 377 nautical mile sector would take 1 hour 3 minutes and need 2.7 tonnes of trip fuel. Route reserve was 0.1 tonne, 5 percent of the trip fuel. The alternate was Bali, 33 minutes away, needing 1.3 tonnes. Block fuel was 7.5 tonnes, giving a takeoff weight of 63.3 tonnes, with 200 kg for taxi.

Start and taxi

Engine start is automatic. Engine 2 started first, and the engine page showed the other engine in amber. The captain joked that they could go to sleep. The A320 can use CFM56-5 or IAE V2500 engines. This aircraft had V2500s, built by a consortium: the fan and low pressure compressor from Japanese Aero Engines, the high pressure compressor from Rolls-Royce, the combustor and high pressure turbine from Pratt and Whitney, and the low pressure turbine from MTU. The V2500 can produce 25,000 pounds of thrust and has an overall pressure ratio of 29.8. On IAE aircraft, thrust is shown as engine pressure ratio, EPR, with a maximum fan EPR of 1.6, unlike CFM aircraft, which show N1 as a percentage.

After pushback from Apron G, the aircraft taxied to runway 07R. The route to Surabaya followed airway W45 through waypoint KASAL, beacon CA, waypoint PIALA, beacons ANY and BA, and waypoint NIMAS. The departure chosen was PURWAKARTA 2C. Flaps were set to 1+F.

Flexible thrust and cost index

Full thrust shortens the takeoff run but wears the engine, and run at maximum for ten minutes, blades would start to melt. On a long runway, the crew enters an assumed temperature higher than the real one so the engine delivers less thrust. That day the assumed temperature was 68 C against an actual 31 C, giving an EPR of 1.301 instead of 1.6. This saves engine life.

The cost index weighs the cost of time against the cost of fuel. An index of 100 means 100 kg of fuel costs as much as one minute of flight time. A higher index favours speed, a lower index fuel savings. Flight 276 used a cost index of 39.

Takeoff and fly by wire

V1 was 154 knots and rotation 155 knots. There is no control column in an Airbus cockpit. The pilot flies with a small sidestick, and the hand that would leave the thrust levers at V1 simply rests on the lap. Pilot inputs go to computers, which then move the surfaces, and the computers limit what the pilot can command. Under normal conditions the A320 will not bank more than 67 degrees, pitch up more than 30 degrees or down more than 15 degrees, or exceed 2.5 G. If a stall is near, it lowers the nose and commands full thrust, and in an overspeed it raises the nose gently.

Climb and a bleed fault

After takeoff, Firman asked for a direct to beacon CA to save fuel and air traffic control agreed. Then the master caution sounded for an engine 2 bleed fault. Bleed air runs the air conditioning and pressurisation. One bleed cannot hold the cabin at 7,450 feet equivalent at FL370, where the pressure is 215 hPa against 768 hPa in the cabin, a difference of 553 hPa. The aircraft would have to descend to about 22,500 feet, where the APU bleed can help. With both bleeds failed it would descend to 10,000 feet, or to 15,000 feet if the APU bleed helped. If the APU supplies only electrical power, the aircraft need not go below 25,000 feet. After a few on and off cycles the bleed worked again well before cruise at FL370.

Aircraft fly high because thinner air means less drag, so higher speeds and lower fuel burn. The flight management system (FMS) builds the flight plan and suggests the most economical climb, cruise altitude, speed, step climbs and descent. With cost index 39, it set a climb at 303 knots until Mach 0.78, then Mach 0.78 to FL370. The changeover happens at about 29,000 feet, called the crossover altitude.

Navigation

The aircraft had two GPS receivers with accuracy listed at 10 metres. Their positions differed by only 0.1 nautical mile, with 10 satellites in view, when the minimum for a stand alone receiver is 4. Three inertial reference units (a 737 has two) also compute position and drift over time, so the FMS updates them from radio beacons. It mixes the IRS position with the two GPS positions to give a very accurate result. On the flight the estimated position uncertainty was 0.08 nautical miles and the required navigation performance on W45 was 2 nautical miles, so accuracy showed as HIGH.

The author contrasted this with Adam Air 574, which disappeared between Surabaya and Manado on 1 January 2007. Indonesia's islands and limited radar make crews rely on manual or automatic FMS updates from VOR and DME, which can be inaccurate. In February 2006 another Adam Air 737 ended up at Tambolaka, about 400 nautical miles from its destination Makassar, after poor VOR updating.

Cruise

At dusk the cockpit lighting changed from blue panels to golden brown. The captain pointed out volcanoes. The FMS first showed an optimum altitude of FL365 and later FL370 as fuel burned off. Flying 2,000 feet above the optimum costs 1.4 percent in specific range and 2,000 feet below costs 2.1 percent. Flying 6,000 feet below costs 12 percent.

Descent and landing

A white arrow on the navigation display marks the top of descent. Firman asked for the descent 30 seconds before it, and the aircraft descended at idle thrust at 2,800 feet per minute. The cabin pressure schedule was set to equal outside pressure at touchdown. The A320 has four flap settings, with two flap surfaces and five slat surfaces on each wing. The captain chose FULL for landing, flaps 40 degrees and slats 27 degrees, with a computed approach speed of 135 knots.

About 30 nautical miles out, air traffic control vectored the aircraft left for traffic, taking it nearly 10 nautical miles off track. The captain was mildly annoyed, since pilots take pride in burning the least fuel. At 10 miles the gear was down with three green lights. Cleared to land on runway 01, the captain disconnected the autopilot with an aural "cavalry charge" and flew manually. The autothrust disconnected at 10 feet with the "retard" call. The touchdown was smooth. He did not use reversers, since the runway was long enough for the automatic brakes at medium, which saves fuel. The APU was started during taxi to power the aircraft at the gate.

It was an ordinary day for the crew. Within thirty minutes PK-RME was ready to go back to Jakarta as flight 277.