“We have both our radio altimeters failed, TCAS failure, ILS is out of service on both sides of planes, no wind shear system either, no auto land, auto speed brake failure and the APU is unserviceable as well”. On 11th September, Captain Rustom Palia piloting AI 101 had to radio this unfortunate catastrophic series of failures to the tower at JFK, New York. While en route from Delhi to New York in a long haul flight of 15 hours onboard the modern Boeing 777-300 ER (Extended Range), the crew faced an unprecedented situation of instrumentation failure compounded with bad weather and low fuel.
Every pilot trains for emergency situations like these but they hope not to see it in an actual operational aircraft in their lifetime. The able team of Captain Rustom Palia, Captain Sushant Singh, First Officer Vikas Singh and First Officer DS Bhatti held their nerve and landed the giant bird with 370 souls on board with no eventuality.
On 11th of September, Air India flight AI 101, a Boeing 777-300 ER, took off from Indira Gandhi International Airport, New Delhi to John F. Kennedy International Airport, New York, one of the elite Air India nonstop service. After flying for 15 hours, the captain started their descend to JFK. However, the weather God had other plans. The weather turned very bad with visibility as low as 200 feet. And to compound the problem, the cloud ceiling also got very low; this means when the aircraft breaks out of clouds, it would be very close to the ground. However, with modern avionics, navigational aid and CAT III B ILS, it should still have been possible to do a safe landing. But to further add to the problems, multiple instruments onboard AI 101 started failing, leading to a potential disaster.
The first instrument to fail was the radio altimeter. Radio altimeter measures the altitude of the aircraft from the ground. It’s a crucial instrument as the output from radio altimeter readings are fed into several other instruments, ILS (Instrument Landing System) being one of the most crucial among them.
What is a radio altimeter and how does it work
A radio altimeter is a simple device consisting of 2 antennas. A transmitting and a receiving antenna. It transmits a radio wave, the wave reflects from earth’s surface and receiving antenna receives it. From the time taken to cover the distance, the altitude is calculated.
For example, say the total time taken is 0.000005 seconds. Radio waves travel at the speed of light. In aviation, distance is measured in feet. The speed of light in feet per second is ~983571087 feet/sec.
Distance = Speed * Time.
983571087*.000005 = 4917 feet.
This is the total distance travelled i.e. from the aircraft to ground and back to receiving antenna on the aircraft.
Dividing it by 2, we get our altitude i.e. 4917/2 ~ 2500 feet.
Instrument Landing System or ILS consists of 2 main component which aids in safe landing of an aircraft. A localiser antenna, which helps in the lateral displacement and aligns the aircraft to runway centre line and a Glideslope antenna telling the aircraft to maintain the glide slope. Consider glide slope as an imaginary line from aircraft to the touchdown zone indicating the height aircraft must maintain at different points till touchdown.
How does ILS aid in landing
ILS consists of 2 components
- Localizer – This is an array of the antenna at the far end of the runway along the breadth of the runway. Every airport has its own ILS frequency. Once the aircraft tunes into a said ILS frequency, antennas to the left of runway centre line emit a particular signal and antennas to the right emit another signal. This helps the ILS receivers in the cockpit to correct course and align the aircraft to the centre line.
Localizer Antenna Array - Glideslope Antenna – This is an antenna installed somewhere on the side along the approach to the runway. This works on a very simple principle of height and distance in Trigonometry we learned during high school. For a smooth and comfortable landing, most airports maintain a 3° downward descend.
Say, the aircraft is at 3 nautical miles from the runway touchdown zone.
p = 6076*3 = 18228 feet
From basic Trigonometry, tanθ = p/b
tan3° = p/18228
0.0524 = p/18228
p = 18228*0.0524
p = 955
So at a distance of 3 miles from the runway, to maintain a glide slope of 3°, the aircraft must be at a height of 955 feet. This information is relayed from the glide slope antenna to the cockpit ILS instruments and flight computers correct the height of the aircraft from inputs that are received from altimeters as well as glide slope antenna. In case the radio altimeters don’t work, this automatic height correction won’t work either.
One failure led to another. As radio altimeter failed, ILS receivers couldn’t work. As ILS didn’t work, auto land and auto speed brake also failed, which meant, the pilots would now have no aid from flight computers and have to do the entire landing manually. To add to the woes, bad weather and low cloud ceiling ensured almost zero visibility. In these conditions, pilots use the most sophisticated landing category called ILS CAT IIIB. But in this case, the entire ILS was missing. The pilots did try to land the plane manually but the poor visibility forced a missed approach and go around. This left very low fuel on board for any alternate airports nearby which had better weather.
At this point, captain Rustom Palia decided to divert it to Newark (EWR), another international airport in NYC Metropolitan area, located in New Jersey. AI 101 asked for permission to land at EWR after describing the ordeal; here is a conversation snippet between Newark tower and captain Palia.
Tower – When you get a chance, can you tell me a number of pax and fuel onboard
AI 101 – We have 370 people and roughly around 7200 KGs of fuel on board. Is there anything else that you need from me
Tower – That will be all.
AI 101 – Thank you very much. We are good. Thanks for this approach
Tower – No Thank you, I wish I could do more for you
With a low cloud ceiling, the crew tried a non-precision approach (VNAV/LNAV). Once they broke out of the cloud, the aircraft was now flying at 400 feet at around 300 KMPH. It was then that the crew physically saw the runway and realised they were close to a touchdown but not aligned yet. Also, the PAPI lights indicated that they were flying too high to miss the touchdown zone. The problem with jumbos like 777 is, they eat up a lot of runway while braking. Hence, they need the complete runway for landing. Touching down beyond threshold can be catastrophic with a runway overshoot. Captain had to snap correct the altitude of the plane, flying at a speed of 300 KMPH, 1.5 miles away from the runway and 400 feet above ground with no automated altitude readout as radio altimeters had failed.
What is PAPI
PAPI stands for Precision Approach Path Indicator. It’s a set of 4 lights mounted on a stand elevated at a particular angle. The lower half of the light emits a Red beam whereas the upper half emits a Whitebeam. The lights are fitted at a particular angle, which aids the pilot visually to correct his height. The angle ensures that at the correct height, the pilot should always see 2 reds and 2 whites. If there are more than 2 Whites, the aircraft is flying higher than the designated height. If the pilot sees more than 2 Reds, the aircraft is flying low.
Newark airport had declared an emergency with rescue vehicles on standby. Eventually, 38 minutes after the first missed approach at JFK, at around 08:43 hours local time, Captain Palia was able to land the plane safely on runway 04R of Newark Liberty International Airport with all 370 people onboard safe. Just to revisit the ordeal, there were multiple instrumentation failures, no navigational aid for flight computers, low fuel, bad weather and zero visibility. Despite all these odds, what the crew pulled off was a miracle of sorts.
While AI has refused to comment on the issue pending an investigation, the crew must be applauded for the amazing feat it achieved.
