It is rather ironical but the Director General of Civil Aviation (DGCA) sent a letter on May 31to CEOs (Chief Executive Officers) of all airlines and aircraft charter companies registered in India instructing them not to haul up pilots for doing hard landings that are within the aircraft manufacturers’ permissible limits. The letter appears to have been issued in the wake of the disastrous end of Air India Express Flight IX 812 while landing at Mangalore in the morning hours of May 22, wherein late touch down to avoid a hard landing was being quoted as one of the contributory factors which led to the aircraft overshooting the runway into a fiery and fatal heap. It had been earlier brought out by one of the national dailies that Air India had hauled up its pilots (those from Air India and Air India Express) for doing hard landings that were technically within the manufacturers’ limits. It is being stated that Captain Zlatco Glusica, the commander of the Boeing 737 which crashed while landing at Mangalore was hauled up in March by the airline’s flight safety department for doing a hard landing amounting to 1.7g (vertical acceleration due to gravity on touchdown but, within the maximum permissible limit of 2.1g) at Thiruvananthapuram.
The DGCA letter goes on to advice, “Pilots need to be made aware that achieving a particular ‘g’ value on touchdown is no measure of a good landing. Landings should be judged not by how soft the landing has been, but if it has been made at the correct speed and touchdown zone on the runway”. Continuing in the advisory mode the letter states, “The airplane manufacturers lay down limits of ‘g’ values for landing and operators need to guard against imposing lower values that may compromise the runway stopping distance required”.
The DGCA’s advice cannot be questioned both in terms of its soundness and correctness, except that it also falls within the realm of ‘basic airmanship’ (read application of common sense in the air) taught to the ‘greenhorn’ student pilots in flying clubs and; not to the highly experienced airlines’ commanders with thousands of flying hours in their respective log-books, to whom this should come as second nature – rooted deeply into their subconscious minds. Actually, when an airplane comes back to mother earth after a flight, its return to the terra firma can be described in a number of ways ranging from a ‘hellish arrival’ to a ‘smooth touchdown’ or a ‘hard thud’ to a ‘kisser’. Any self-respecting pilot always aims for a smooth touchdown, however without compromising on the first two vital ingredients of a good landing, i.e. correct speed and the correct touchdown zone.
On final straight-in approach to landing, a pilot generally controls the airplane speed with throttle(s) and aims for the correct touchdown zone by staying on the correct glide slope either visually (assisted by VASI/PAPI etc) or by electronic means (following GCA controller’s instructions/ILS etc). Having arrived correctly on the runway, the flare is commenced, i.e. the nose of the aircraft is raised by gently pulling back on the control column in a progressive fashion in order to arrest the vertical speed of the airplane for a smooth touchdown. A ‘kisser’ landing results when the airplane is controlled in such a manner so as to achieve zero vertical speed precisely at the time of the wheels coming in contact with the runway surface. A landing of this type would be perfect provided the airplane touches down at the correct speed and within the touchdown zone (generally 200-400 yds up from the landing runway threshold). It would be interesting to note that the USAF actually has a system of drawing a line on the runway (approximately 1,000 ft from the threshold) and a smooth touchdown on the line could earn the pilot a perfect ‘A-Spot’ grading. Even the IAF has similar system of grading the landings, especially, during competitive exercises. A landing of this type would have a 1’g’ vertical force or even less with the weight of the aircraft slowly descending on the tyres due to gradual reduction in lift – as the speed of the aircraft drops after touchdown. Sometimes, the passengers may not even feel that the aircraft has landed till they become aware of the ground roll. It may be history now but during the Maharaja’s hay days, Air India was renowned (more as a rule than exception) for its ‘A-Spot’ landings.
Notwithstanding the above, there are occasions when the aircraft are deliberately landed with certain amount of vertical speed – termed sometimes as ‘positive landing’. These would generally be resorted to when the runway is wet (to prevent hydroplaning) or when there is a need to go in for ‘short’ landing (runway length smaller than desirable, sluggish brakes, etc). This is where ‘g’ limits for landing come into play. For example, when Boeing clears B737 to a landing limit of 2.1g, it means that the undercarriage is strong enough to withstand vertical loads of 2.1 g without any damage. All this must have been known to the aircrew of the ill-fated IX 812 who, when faced with a ‘high & hot’ (greater height and more speed than desirable) being brought out by the media as initial findings, would have opted for a deliberate ‘positive’ (read hard) landing or better still, for a go around to make another, safe approach.
Landing Technique
View Point: Air Marshal (Retd) B.K. Pandey
The impact force experienced by an aircraft when it makes the first contact with the runway surface in the process of landing is measured and expressed in terms of ’g’ value. For every aircraft type, there is an upper limit for impact force on touchdown at the maximum landing weight defined by the Original Equipment Manufacturer (OEM). In case this limit is exceeded while landing, the aircraft would have to be taken off line and subjected to airframe inspection and repair of any damage that might have been sustained. This would obviously lead to loss of revenue for the airline.
Normal technique of landing requires the pilot to make a steady approach and cross the runway threshold at not more than 50 feet above the ground at the prescribed speed, flare out i.e. establish a flight path parallel to the runway surface few feet above the ground and maintaining the landing attitude, allow the aircraft to sink under control to make firm contact with the ground on the main wheels. In the process the aircraft will float over the runway surface for some distance but in all probability, will land in the touchdown zone. However, if the pilot is focussed on making a ‘soft landing’ as against a firm contact, he would attempt to minimise the sink rate after flare out and in the process unnecessarily extend the float thereby delaying touchdown. In an effort to execute a ‘soft landing’ under pressure of the airline management or the DGCA , even after a normal stabilised approach, a pilot could end up using up more runway length than required for touchdown thus compromising on stopping distance available with possible disastrous consequences.
Depending on the type of aircraft on its inventory, the airline lays down impact force limits for normal operations as also the checks to be carried out on the aircraft and requirements of documentation if the laid down limits are exceeded. ’g’ limits beyond which pilots are to be counselled or subjected to corrective training are also specified. The ’g’ value defining the dividing line between a s ‘ft landing’ and a ‘hard landing’ is also specified and pilots are constantly under pressure to achieve a ‘soft landing’.
The landing ’g’ limits were defined with the long term objective of limiting cumulative stress. However, to enhance passenger comfort, the DGCA had issued a circular in the recent past directing airlines to report all cases of landing where the ’g’ value recorded is greater than 1.8g. It is understood that airlines have lowered the maximum impact force limit further aggravating psychological pressure on the pilot in command. It is rumoured that Captain Glusica, the Commander of the ill fated Air India flight IX 812 had been hauled up by the airline in the recent past on this account and hence would have been under pressure to achieve a soft landing despite being higher than normal on approach.
In the aftermath of the Mangalore tragedy, it appears that the horrendous implications of the practice of enforcing unrealistic limits to landing ’g’ values well below those prescribed by the OEM, have finally been understood by the regulators. Without any dithering, the DGCA has issued fresh directives on the subject to all airlines. The new circular places the situation in the correct perspective by qualifying a good landing as “not one that the passengers perceive as a soft landing, but one that is made at the correct point on the runway with the correct flight parameters”. The circular adds “an attempt to cushion descent to make a soft landing, could result in a delayed touchdown that would need harsh deceleration to maintain the aircraft on the runway or even worse a runway ‘overshoot’ with possible catastrophic results”.
The ground realities have dawned on the Regulator though a little late. But better late than never!
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