by on June 27, 2020
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Air Carrier land into Imphal air terminal are right now under investigation by the Directorate General of Civil Aviation (DGCA). The inquiry being examined is: during decent to land in Imphal, do certain pilots break the cutoff points set for a protected decent rate to wilfully complete an "unstabilized" or risky approach and landing so as to save fuel. 

Early this week, Capt Amit Singh, an air security expert sent a mail to the DGCA making the controller aware of a specific example that he had seen in Imphal landing, did among January and June 2020. "An aggregate of 28% of flights fell in the high-risk classification with high vertical speed (rate of descent) more than the safe limit of 1000 feet/min, when the airplane were under 1000 feet over the ground level," his letter said. 

Reacting to his email, executive general of DGCA, Arun Kumar stated: "We are leading a point by point actuality discovering investigation into this matter and its security implications. Based on the result, further activity will be started." 

These discoveries have a catch however. Capt Singh said he sourced information from flightradar24, an application that tracks flights ongoing. "The information might be inclined to blunders and errors when compared  with the airplane flight recorder information, therefore I would request DGCA to analyse the airline’s flight recorder data," he said adding that, to eliminate large scale inaccuracies he had used a single source and followed multiple airline flight operations.

To discern possible patterns, he plotted the vertical speed of aircraft of three airlines when flying between 800-500 ft above ground level. He found that few flights were probably using a 'lower flap setting' for landing. "The data shows a consistent high rate of descent and ground speed which is normally associated with lower flaps settings that are used for fuel savings," he said.

 

 

When flaps are extended, the wing produces more lift, which is the upward force that keeps an aircraft airborne. The wings of an aircraft flying at, say 150 knots with no flaps extended, will produce considerably lower lift force than that of a similar aircraft flying at the same speed with fully extended flaps. An aircraft with flaps deployed can keep itself airborne at much lower speeds than one with a clean wing.
But the flaps don’t help wings produce extra lift for free. When flaps are lowered the drag induced on the wing increases, the aircraft speed reduces and it consumes more fuel to fly a given distance. Flaps are then extended only during take off and landing, when an aircraft needs to be air borne at lower speeds or in case of a landing, come in to land at lower speeds without risking a stall.
Approach and landings can be carried out with fully extended flaps or a partly extended one (lower flap setting or a flap 3 setting). An aircraft with a lower flap setting will consume lesser fuel to fly a particular distance than one with full flaps. But a lower flap produces lower lift than full flaps. To compensate for the lower lift, aircraft with a lower flap setting has to carry out an approach and landing at a comparatively higher speed (than one with full flaps), if it intends to stay airborne.

 


At most airports, in certain weather conditions, landing can be carried out safely with a lower flap setting, with aircraft coming in at comparatively higher speed. The advantage of a lower flap setting (as compared to a full flap) is that it results in fuel savings, lower carbon emissions and makes for a greener landing. Airline pilots proficient in carrying out a stable approach and landing with a lower flap setting are a commendable lot. The question is, how should passengers view the pilots who carry out an unstable approach and landing with a lower flap setting?
To begin with air safety experts opine that a lower flap setting is unsafe when landing in certain weather conditions in airports like Imphal. What is peculiar with Imphal? Here’s the low-down. Imphal airfield lies in a valley, surrounded by hills, forcing aircraft to follow a comparatively steeper angle of descent when coming in to land. Imphal runway 04 is installed with an Instrument Landing System, making it the preferred runway despite tailwind on approach and landing.

 


Given the terrain, Imphal runway 04 "glide-slope of the instrument landing system" (it’s a ground-based navigation aid that relays radio signals to the cockpit which provide descent-path guidance upto the runway) is set at a 3.5 degree descent angle, as against the 3 degree glide path angle commonly found at most airports. The 0.5 degree increase in descent angle adversely affects certain parameters of approach and landing, like descent rate, speed etc unless the aircraft is configured (with full flaps, for instance) to keep these criteria within safe limits. A steep descent by itself increases the aircraft speed, add to that a lower flap setting and what you have is an aircraft coming in high and fast. "Such approaches carried out in strong tailwind conditions and/or wet runway could lead to a hard landing or a runway excursion," said Capt Singh.

 


There are enough studies, papers published worldwide on the increased risks brought on when landing at airports with a glideslope of 3.2 degree or higher in tailwind with a lower flap setting. A US Federal Aviation Administration safety bulletin states "at 3.5 degree glide slope angle, there is almost two times likelihood of an unstable approach. If its with flap 3 and in tail wind conditions, then the probability is much higher."
Even without a lower flap setting, landing on a runway with high glide path angle calls for care. An International Federation of Air Line Pilots Association (IFALPA) paper giving IFALPA’s position on tail wind operations states: "Operators should support not conducting tailwind approaches on steep approaches or Glide Path greater than 3.0 degrees." Another IFALPA paper that discusses vertical approach profile states: "Approaches steeper than 3 degrees have safety, operational and capacity concerns. Safety factors include increased pilot workload, reduced path stability, lack of harmonized glidepath angles, different flare characteristics, landing point dispersion, increased risk of hard landing and tailstrike, delayed engine spool-up, and increased influence of atmospheric circumstances (especially tailwind, temperature and icing conditions)."

 


It’s logical to wonder then, why hasn’t a runway excursion occurred yet at Imphal airport, if, according to flightradar24 data, one in four approaches carried out in the past six months was an unstabilised approach, with a higher than normal descend rate and ground speed. Which means, even though the aircraft came in high and fast, the pilots could bring the aircraft to a halt within the runway (data on hard landings, if any, would only be with the airline).


Accident causation models, especially the Domino (H.W Heinrich) model could answer that. One of the axioms of the Heinrich model is: "The person who suffers an injury (pilot, in this case) has probably had at least 300 narrow escapes when committing the same act.” Accidents are the result of a chain of interactions, sequential events, with each event triggering the next one, it says. "Removing one of the key factors (event-domino) can break the link-sequence."

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Divya Kapoor
The report states that certain airlines and/or aircrews are leaving flap selection for landing as late as possible in an effort to save fuel/time. So aircraft are descending to the runway too quick and too steep by leaving flap selection as late as possible which in most cases means the aircraft is ... View More
June 27, 2020