The wreckage of VT-PHF, a Pawan Hans-operated Mil Mi-172, is collected ready for accident analysis. Accident Investigation Bureau Photo

features Understanding helicopter performance

In 2011, 19 lives were lost when a Mil Mi-172 crashed on approach to a helipad 
in a mountainous part of northeast India. The resulting investigation posed 
some troubling questions.
Avatar for KP Sanjeev Kumar By KP Sanjeev Kumar | July 14, 2020

Estimated reading time 11 minutes, 33 seconds.

On April 19, 2011, Pawan Hans Ltd. (PHL) was operating a Mil Mi-172 helicopter (VT-PHF) on the Guwahati-Tawang sector in northeast India. While on short final approach to the high-altitude Tawang helipad, the helicopter, carrying four crewmembers and 19 passengers, descended below the level of the helipad, hitting its concrete edge and toppling over. The resulting crash and fire claimed 19 lives, including two crewmembers.

The wreckage of VT-PHF, a Pawan Hans-operated Mil Mi-172, is collected ready for accident analysis. Accident Investigation Bureau Photo
The wreckage of VT-PHF, a Pawan Hans-operated Mil Mi-172, is collected ready for accident analysis. Accident Investigation Bureau Photo

Following the crash, the Aircraft Accident Investigation Bureau (AIB) found there had been nothing wrong with the helicopter or its stated performance before the incident. However, the elevation of Tawang helipad (8,250 feet above mean sea level), combined with an outside air temperature (OAT) of 16 C, marked a density altitude of approximately 10,000 feet, and this was a significant factor. No particular approach profile specific to the category of rotorcraft was flown, and the helicopter was overloaded.

A careful reading of the investigation report reveals two major aspects which coalesced into that accident: incorrect or negligent performance calculations; and a host of individual, organizational and other latent failures that aligned like the proverbial holes in Swiss cheese.

Those who survived the crash landing were asphyxiated by the thick smoke, with all but four of them enduring a slow and painful death. The pilot, who survived, faced charges for negligence. The co-pilot died in his seat.

The desire to avoid such a fate should be motivation enough for a deeper understanding of helicopter performance, and that’s what we’re going to focus on here.

What went wrong?

VT-PHF, with twin turboshaft engines and a maximum certificated all-up mass of 26,455 pounds (12,000 kilograms), held an airworthiness certificate from the Indian Directorate General of Civil Aviation (DGCA) as a Category A helicopter. As per the service contract, it was required to be operated under Performance Class I, which would provide adequate safeguard even if an engine failed at a critical juncture during takeoff or landing. Yet, with two fully operational engines and all systems “green,” the helicopter crashed. Why?

The accident investigation revealed that at an estimated landing weight of 24,070 lb. (10,918 kg) the helicopter was at least 1,320 to 1,545 lb. (600 to 700 kg) “above the prescribed AUW [all up weight] limit [sic] for Tawang” at 16 C. This, and another observation in the report that “in any case this weight is within OGE [out of ground effect] configuration but beyond the single-engine limitation at that altitude,” miss standard phraseology such as regulated takeoff weight (RTOW) and OGE hover ceiling.

But does operation “within OGE configuration” ensure Category A performance? Which single-engine limitation are we talking about? Is Category A certification meant to protect us in an all engines operative (AEO) condition like this one? Many questions raised by this accident have answers that remain fuzzy in the minds of many helicopter crews and management.

The International Civil Aviation Organization (ICAO) Annex 6 (Part III) contains standards and recommended practices for all commercial helicopter air transport operations.

Every helicopter produced today is certified either as Category A (Cat A) or Category B (Cat B). The distinction is important. As per an accepted definition, “Cat A means a multi-engine helicopter designed with engine and system isolation features capable of operations using take-off and landing data scheduled under a critical engine failure concept which assures adequate designated surface area and adequate performance capability for continued safe flight or safe rejected take-off.”

When Cat A helicopters are operated in conformity with laid down profiles and performance charts in the rotorcraft flight manual (RFM), it ensures a guaranteed stay-up capability in the event of a critical engine failure.

Accident investigators discovered impact marks on the edge of the helipad that had been created by the helicopter's main rotor blades. Accident Investigation Bureau Photo
Accident investigators discovered impact marks on the edge of the helipad that had been created by the helicopter’s main rotor blades. Accident Investigation Bureau Photo

Cat B, on the other hand, means a single-engine or a multi-engine helicopter that does not meet Cat A standards. Cat B helicopters have no guaranteed ability to continue safe flight in the event of an engine failure, and a forced landing is assumed.

Performance classes I, II and III indicate, in a decreasing order, the capability of the rotorcraft to safely land or continue flight in the event of an engine failure. Performance class III, for example, means the helicopter will have to force-land in the event of engine failure. All single-engine helicopters come under this classification. Helicopters operated in Performance Class 1 are required to be certificated under Category A.

The profiles and performance figures for Cat A, though flown with AEO, are relevant to a one engine inoperative (OEI) condition during takeoff or landing. It is possible to wreck even a Cat A certified helicopter through mishandling or overloading (or both), as the Mi-172 example indicates. Cat A profiles may require the use of OEI contingency rating if an engine fails, without which safe recovery is not ensured.

A lot of work has been done by the industry to build these safeguards. In the field, diligent planning and strict adherence to stipulated profiles are required to ensure that this work is not waylaid by deviations in operating procedures.

The importance of pre-flight calculations

Contracts that require operations under Performance Class 1 need adherence to Cat A regulated takeoff and landing weight limits. Operation within “OGE configuration” does not necessarily equate to Performance Class 1.

Sadly, the approach profile of the ill-fated Mi-172 was untenable by the aerodynamics of a helicopter landing at a high altitude helipad with up to 1,545 lb. of excess mass, even with two engines turning at maximum power. Unfortunately, there was little evidence that helicopter performance was given due importance on that fateful day or in the run-up to contract finalization. The location and operating conditions amplified these mistakes.

For Tawang helipad with OAT of 16 C at the time of accident, the rejected takeoff (RTO) distance for VT-PHF’s “assumed” mass of 22,490 lb. (10,200 kg) was 885 feet (270 meters) — as determined by the AIB from the Mi-172 RFM. Yet the helicopter was operating to a “table-top” helipad with a prepared area that was less than 330 feet (100 meters) with no clear final approach and takeoff (FATO) area. By all accounts, the area of operation met the definition of a “hostile” environment.

Performance Class 1 is meaningless without taking into account the area and type of operations. Clear areas for baulked landing (overshoot), continued takeoff (CTO) or RTO cannot be overlooked just because a helicopter is certified under Cat A.

Also, bear in the mind that a Cat A approach is flown with all engines operative. It gives you the best transition to a Cat B landing should one engine fail on approach.

To the Mi-172 crew’s misfortune, the accident occurred with both engines turning, where no specific profile was flown — even though they were operating over hostile terrain.

Investigators take a look at the accident aircraft's port engine. Accident Investigation Bureau Photo
Investigators take a look at the accident aircraft’s port engine. Accident Investigation Bureau Photo

Understand performance

Numerous helicopter accidents can be traced back to insufficient or incorrect understanding of how the machine interacts with its environment. Good helicopter pilots will always respect WAT — weight, altitude and temperature. Winds can either add or subtract from this, so a healthy understanding of wind effect on performance is also a must.

Weight is well understood. Altitude and temperature determine density altitude, which is what the propulsion system “sees” or “feels” on any given day. In addition, rotor speed may also be a determinant for helicopters that have selectable rotor speed for certain maneuvers such as Cat A takeoff/landing profiles.

The helicopter is a complex machine kept in the air by a number of rotating devices that have their own mechanical and aerodynamic interplay. The engine(s), gearbox and rotor system that together form the propulsion system foot the bill for any “performance checks” signed by the pilot. Many airflow interactions between main rotor and tail rotor, wind and fuselage, empennage, weight, altitude, temperature and environmental peculiarities dictate the machine’s performance and handling on any given day.

A helicopter’s RFM usually contains four to five sections. It usually follows a logical sequence: limitations, normal procedures, emergency or abnormal procedures, performance, and weight and balance. There may be additional sections or “supplements” that cover special procedures (such as Cat A operations), optional equipment, or enhanced performance.

But all this constitutes just heaps of paper should one decide to ignore it. There is no safety in flying by “feel” as far as performance is concerned. It’s a deadly mistake to treat graphs and performance figures casually.

The maximum takeoff weight (read mass) of a helicopter proceeding for a mission on any given day is usually defined by one of several published limits: the maximum certificated all-up weight; the regulated takeoff weight as per weight, altitude, temperature (Cat A performance); the maximum permissible weight for hovering IGE or OGE; the maximum regulated landing weight at your destination; any other overriding condition.

The first one is fixed and inviolable. The others may change depending on the conditions and type of operations. These figures are used by crew to determine if the actual takeoff weight lies within published limits. The tendency to loosely interpret them or use such limitations interchangeably has imperilled many helicopters. For instance, IGE/OGE hover ceiling and MTOW cannot be used interchangeably.

You need either a power reserve, height, or runway length to perform a takeoff without exceeding aircraft limits. On a hot day, taking off from a helipad with a helicopter loaded to its IGE hover ceiling means you’ll either have to do a rolling takeoff, use ground cushion and translational lift to gain height, or lose height to gain speed and lift. If there is no room for such a maneuver, be prepared to trim the treetops as several crashed helicopters have unsuccessfully demonstrated.

It is crucial that the correct graphs are chosen for reference. The conditions for which the performance is applicable will be denoted on the header or footer of the graph. If those “box conditions” are not respected, performance cannot be guaranteed, nor would it be legal to use that graph. If you are loaded over the Cat B limits and you do not adhere to the Cat A profiles or performance charts for takeoff or landing, an “exposure window” is inevitable.

Maintaining control

Some industry experts have argued that the importance given to Cat A certification and all that it entails glosses over other critical vulnerabilities such as continued visual flight rules (VFR) operation into instrument meteorological conditions (IMC) — a bane of all helicopters. They feel a realignment of priorities is needed to balance the increasing reliability of turboshaft engines with investments to improve the vulnerability of being inadequately equipped for instrument flight rules (IFR) operation. Helicopters are lost to controlled flight into terrain (CFIT) and loss of control (LOC) at a sickening regularity.

Thick black smoke quickly enveloped the Mi-172 after it hit the helipad. Accident Investigation Bureau Photo

Excluding CFIT and mechanical failures, piloting technique and performance accounts for a major share of helicopter accidents. A study from the International Helicopter Safety Team determined that LOC figures in one out of every five fatal accidents. Low visibility and LOC taken together contribute to one-third of all fatal accidents in helicopters.

A final example bears testimony to this statistic.

Eleven days after the Mi-172 crash, another PHL helicopter crashed in the hills after taking off from the very same location — Tawang. The single-engine Airbus AS350 B3 (Performance Class III) had taken off from Guwahati to Tawang to meet an airlift requirement for the chief minister of Arunachal Pradesh.

The AIB observed that “the pilots were in a hurry,” missed some mandatory R/T calls and flew a positioning flight of approximately 50 minutes with “poor flight planning” prior to landing at Tawang. After the positioning flight, they switched off, refuelled, started up again, and repositioned on the same helipad to embark the chief minister and his team — all within 14 minutes. On initial lift-off, the helicopter yawed 70 to 80 degrees due to overloading. It took the crew three attempts — each time offloading a passenger or cargo — before they could finally get airborne.

We can only speculate as to what performance calculations must have gone into such a flight. But in this case, the end came from continuing VFR flight into IMC conditions. The helicopter slammed into a steep hillside soon after takeoff with no survivors.

Helicopters often get picked for missions by people who have no idea of the nuances of rotary-wing flight. Because of this, it is vital that crews develop a sound understanding. Learn to say “no” when the situation demands. Drop that extra passenger or piece of cargo. If you are barely able to hover, how will you ever take off without having either height or distance to get through the “hump” of translation? Will you be able to protect your passengers from an engine failure?

Be safe. Make sure you have done your numbers.

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