Can a pilot press a wrong button in a cockpit causing the plane to crash?
Pilots are highly professional with thousands of hours of training on ground, simulator and aircrafts before they get to fly a commercial plane. There are redundancies built into the flight systems. Aircrafts have warning mechanisms. The ground staffs constantly observe all aircraft movements.
Yet this happened on 14th February 1990 with an Indian Airlines aircraft, Flight 605 from Mumbai (Then Bombay) to Bengaluru. Flight 605 was a scheduled flight between Mumbai’s CSIA to Bengaluru’s HAL airport on an Airbus A320.
On the fateful day, it was being piloted by CA Fernandez who had 9037 hours of flying out of which 68 hours was on A320. Capt. Fernandez was undergoing the first of 10 route checks required for qualification to Captain. He was being supervised by co-pilot and check pilot SS Gopujkar.
Capt. Gopujkar had 10,340 hours of flying experience out of which 255 hours was on A320.The aircraft left Mumbai at 11:58, after an hour delay, with 139 passengers and 7 crew members. The take off and cruise phase of the flight proceeded normally. At around 12:25, the aircraft began its descend into Bengaluru and contacted Bengaluru approach control. The approach control let the pilots know about visibility, weather and wind all of which were normal.
After about 15 minutes, the flight was cleared to descend to flight level 110 (FL 110 = 11,000 feet). At 12:53, IA Flight 605 appeared on Bengaluru’s radar blip. At this point, the approach control handed over flight 605 to Bengaluru Air Traffic Control tower who would lead the aircraft to the ground.
Tower informs the pilot to approach on Open Descent mode, make a right turn and make a visual approach to Runway 09. A bit of explanation here.
During descend, aircrafts can select either of the 2 modes. Open Descent vs Vertical Speed.
Open descent (OP DES) is when the aircraft begins its approach phase. You must have heard the pilots saying, “Ladies and Gentleman, we will shortly commence our descend to Mumbai”. That’s when the pilots engage the open descent mode.
In this mode, () the aircraft goes to idle thrust i.e. the engines are running at ideal RPM. Imagine your car going down a hill. At most times, you take your feet off the accelerator pedal but the car still maintains the speed because of the downhill. If you want to control the speed, you would apply brakes or change the pitch of the road lowering the downhill angle (No you can’t do that hence you have to apply brakes).
OP DES mode is something similar. The Auto Flight System (AFS) commands idle thrust and the elevators adjust the pitch to maintain the target speed. The pilot inputs the speed and the AFS pitches up (slows down the aircraft) or down (speeds up) the aircraft to maintain that speed at idle thrust.
Open Descent mode is selected to lose altitude fast i.e. say from 10,000 feet to 1000 feet. It’s really simple. You are at 10,000 feet and want to go down to 1000 feet. You let the engines in idle mode and allow the aircraft to come down. Once you are at 1000 feet at final approach, you can’t be in open descent any more. From here on, it has to be guided descent with adequate engine power.
Imagine you are at the fag end of the downhill and your destination is on sight. Ideally you won’t leave your engine at idle and would like to take control. That’s when pilots switch to Vertical Speed mode.
Vertical Speed (V/S) mode is chosen during final approach i.e. when the aircraft establishes itself on glide slope (GLS). Unlike OP DES, in V/S mode, the AFS manages both pitch and thrust level. Once on final approach, the standard angle of descent is 3 degree. Aircraft can’t descend at idle power because the aircraft needs to maintain a guided profile in order to touch down the runway at the right place. You can’t have a target speed here. Your speed has to change constantly so as to maintain the aircraft on the GLS.
The picture above tries to describe the difference between OP DES and V/S. The picture is for representational purpose only. Aircrafts don’t descend at such steep angles. The altitude and distance to runway are made up figures not to be taken at their face value. All that the pilot has to calculate here is, when to start their descend so that they can kill enough altitude and distance to join the glide slope at 1000 feet altitude and 5 mile distance from RWY. This is called the descend profile of an aircraft. The point at which it must start descending is Top of Descent or T/D as shown in the picture. A wrongly calculated descend profile often results in runway overshoot or early touchdown. Remember the Mangauru incident involving at Air India express? Well, another article some day. We have given a more mathematical calculation involving simple trigonometric height and distance concepts here.
Now back to Flight 605. After being cleared for RWY 09, at around 5000 feet, Fernandez realises they are 600 feet above the glide slope(GLS). To rejoin the glide slope, Fernandez asks for a go around. It would mean that, they would climb again to 6000 feet, go around, come back and rejoin the GLS at normal altitude. At this point, Gopujkar asks, “Do you want a go around or vertical speed?” Fernandez choses the latter and asks for a descent rate of 1000 feet/min, higher than the normal 700 feet/min. They now go to V/S mode where flight computers are managing pitch angle and engine power for guided speed to descent at 1000 feet/sec. This is the right mode for landing once you join glide slope. After they joined the GLS, captain Gopujkar asks captain Fernandez to put it back to 700 feet/sec.
Capt Fernandez, however, instead of choosing the vertical speed knob, choses the altitude knob. The knobs are located next to each other and were similar in design. That could have led to the confusion. So the pilots, instead of asking the aircraft to descent at 700 feet/min asked it to descend to 700 feet. This action switches back the aircraft from V/S mode to OP DES again which means engines are now at idle power. However, both pilots are unaware of it.
With minimal engine power, the plane starts falling first. At lower altitude with slats and flaps extended, due to lack of engine power, there is not enough lift to keep the aircraft flying. The radio altimeter gives a call out at 500 feet. This is when captain Gopujkar realized, something is not normal. He quips, “So you are descending on idle power all this time, eh?” At this point they tried switching off the flight directors, an autopilot system that displays a guide on the artificial horizon and attitude, which helps the pilots to fly manually or auto pilot to fly the plane as per the guidance. Had this been switched off, the plane could have sensed that it was dropping fast and could have gone to speed mode to give enough engine power for a possible recovery. But, flight directors were not put off.
At 135 feet, both pilots finally realized the gravity of the impending disaster. Fernandez screams, “Hey we are going down” to which Gopujkar exclaims, “Oh shit”. Last recorded voice on the CVR. Fernandez immediately orders a TOGA (Take-off Go-around) and applies full take off power. But it was too late by then. The aircraft’s nose pitches up but the aft part of the fuselage hits Karnataka Golf Club’s ground, 2300 feet short of BLR runway, 200 feet right to the imaginary centre line.
The aircraft rolls to left and then lifts to 230 feet again. Descends again and hits the ground where the landing gears dug into the ground and aircraft hits a 12 feet high embankment. Investigators suggested that had they pulled the throttle 2 seconds earlier, the disaster could have been avoided.
Immediately after the crash, fire starts in the aircraft. Some passengers jump out of a break that had opened in the fuselage. Crew members helped passengers disembark. However, due to an unserviceable radio transmitter between Bengaluru ATC and fire station, fire brigades could not arrive and passengers who survived the crash, burnt to death. From 146 passengers and crew members, only 54 survived. Official investigations by DGCA as well as Airbus blamed pilot errors to be the cause. Indian Commercial Pilot Association however disputed this. They blamed erroneous design by Airbus to be the cause. They claimed that the engine went into idle power because of a serious flaw in Airbus’s Fly-By-Wire (FBW) automation system.
Less than 2 years after the crash, a similar incident occurred in France involving another A320 which forced the Airbus engineers to change the design of both the knobs and add two preceding Zeroes (00,XXX) to the vertical speed display to avoid confusion.
Though such an incident is not possible in today’s time due to improved instrumentation, it goes on to show how a small mistake can lead to a disaster with catastrophic consequences.
This post was last modified on September 29, 2019 1:50 pm