Understanding Helicopter Performance

What Happened?

On 19th Apr 2011, Pawan Hans Ltd (PHL, then PHHL) was operating a 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 with 4 crew members and 19 passengers descended below the level of helipad, impacted with the concrete edge and toppled.

The crash claimed 19 lives, including 2 crew members and an aircraft maintenance engineer (AME) of PHL. The investigation found nothing wrong with the helicopter or it’s stated performance. The elevation of Tawang helipad (8250 feet above mean sea level), combined with outside air temperature (OAT) of 16ºC, marked a density altitude of approximately 10000 feet. This was found to be 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
  • A host of individual, organisational and other latent failures that aligned like ‘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.

If that’s motivation enough for a deeper understanding of helicopter performance, please read on further. We will focus on the first aspect alone in this article.

What Went Wrong?

VT-PHF, with twin turboshaft engines and a maximum certificated all-up mass of 12,000 kilograms (26,455 pounds), 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?

Mi-172 Crash at Tawang 19 Apr 2011 (photo from AIB report)

The accident investigation revealed that at an estimated landing weight of 10,918 kg (24,070 lb.) the helicopter was at least 600 to 700 kg (1,320 to 1,545 lb.) “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.

Categories of Rotorcraft

The International Civil Aviation Organization (ICAO) Annex 6 (Part III) contains standards and recommended practices (SARP) 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.

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 safety is not waylaid by deviations in operating procedures.

An IAF Mi-17V5 at a high altitude helipad in the Himalayas (Picture courtesy IAF)

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 700 kg 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 10,200 kg was 270 meters (885 feet) – 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 100 meters (328 feet) 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 Reject Takeoff (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.

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 cheques” 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.

A naval ALH Dhruv helicopter over sea (Picture courtesy INAS 322, Indian Navy)

It’s a deadly mistake to treat graphs and performance figures casually

Understand Weight & Balance

The maximum takeoff weight (MTOW, 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 (RTOW) as per weight, altitude, temperature (for Cat A performance or any other reason); the maximum permissible weight for hovering IGE or OGE (hover ceiling); 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 above 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.

As an example, if you refer the ‘drop-down height’ carpet graph for Cat A elevated helideck departure from the Bell 412 EP RFM and chose not to beep up the RPM to 103%, performance as per the graph may not be achievable.

My Moment of Reflection – ‘Mission WGC’

A forenoon spent at Wellington Gymkhana Club (WGC) nestled in The Nilgiris (Blue Mountains), a salubrious hill station in southern India, prompted me to run this jigsaw puzzle through my mind. I recount this example to illustrate nuances of helicopter performance that we take so much for granted.

An Indian Air Force Mi-17 landing at the Wellington Gymkhana Club grounds in The Nilgiris (picture by Kaypius, 2018)

Consider this IAF Mi-17V5 tasked to ferry a VIP from an Air Force Station (AFS) at sea level to a helipad in the hills. The flight entails climb to a cruise altitude that allows adequate enroute clearance from mountains, cross few ridges in the Nilgiris, & finally arrive at the helipad located on the golf course at WGC.

Since a VIP is embarked aboard, the military crew and aircraft have been chosen to meet Performance Class I requirements (an assumption for the purpose of this story).

The takeoff from AFS is the easy part since the departure is from a runway almost at sea level. The AUW for that takeoff would likely not be the determinant for MTOW that day.

Having done a ‘clear area, Cat A’ departure from AFS, the helicopter now commences its climb to bridge the short distance between AFS and Defence Services Staff College (DSSC) in the hills. There would be a certain climb gradient required. Easy as the breeze for a helicopter designed to deliver ‘hot & high’ performance at Himalayan heights?

Ah! Now take an engine out of the calculation.

The twin-engine helicopter is now down to one engine just as it scales the first ridge. A well-prepared crew would have foreseen and planned for such an exigency by looking up key parameters like single-engine rate of climb, climb gradient on single engine, drift-down altitude, etc. Remember, Cat A performance graphs and takeoff/landing profiles are meant only for takeoff and landing. It ensures no safety for enroute engine failures where terrain clearance could impose other restrictions. How many of us delve into such calculations?

Now let’s give them back the engines and proceed with our flight.

The helipad on golf course WGC sits in the middle of a fairway, hemmed-in by hills on either side. The approach is unidirectional, meaning that winds may not always be of your choosing. ETA is close to midday when temperatures can touch early 30s, sending the density altitude soaring upwards. Approach has to be steep, commencing just after scaling a ridge and ending a mile up the fairway. Go-around has to cater for hills in the take off direction.

Bisons on the fairway at Wellington Gymkhana Club (Picture by Kaypius)

There are power lines running between hills on either side, bisons grazing on the fairway. Rolling takeoff is ruled out. It has to be a hover departure. To complicate matters, the VIP entourage disembarking at WGC would be replaced with a return payload that needs ‘urgent repairs’ at Coimbatore, down in the foothills. Simply put, we need a Performance Class 1 arrival at WGC but can accommodate a Performance Class II departure from the same helipad.

What graphs would you refer to complete this calculation? Maybe the ‘Helipad’ or ‘Short Field’ Cat A graphs and profiles? The regulated landing weight for Cat A would give us adequate safety should an engine fail on approach to landing – but with important caveats. You will have to account for the height loss in completing both the baulked landing and continued takeoff manoeuvres – and ensure there is adequate room for that. Nobody but you as crew can determine that before the flight. If there is an iota of doubt, consider your options, including dropping a pax or two from the manifest at point of departure (it’s too late to drop them after takeoff).

Transition from ‘all engines operative’ flight to ‘one engine inoperative’ will involve a height loss unless power available exceeds power required. You may have conquered the plains in this calculation, but the hills can test you. Make sure all ends are secured. Then go for it as per a plan (many RFMs stipulate such profiles for planning).

If a Cat B departure has been chosen, bear in mind that there would be an ‘exposure’ window as you climb out with that payload. Losing an engine during the takeoff may entail a force landing, with slightly worse consequences than a CTO. Do remember to look out for wires and other obstacles, as your flight path after reverting to OEI may not give you the required climb gradient for safety.

If you have asked all these questions and referred the appropriate performance charts, chances are, you will not be caught unawares – provided you respect the profiles stipulated in RFMs and ‘fly by numbers’ as much by ‘feel’. To simply pick up collective & tuck it underarm, ease the nose forward, hope like hell things work out –  that’s a throw of dice; you may end up on the wrong side of an accident investigation board someday.

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.

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.

Same location, 11 days later, another accident

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 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 overloading. It took the crew three attempts – each time offloading a passenger or cargo – before they could finally get airborne.

We can only speculate on 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 entered clouds and slammed into a steep hillside soon after takeoff. There were no survivors.

Do the numbers

Helicopters often get picked for missions by people who have no idea of the nuances of rotary wing flight. Therefore, 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 a critical power unit failure?

Be safe. Make sure you have done your numbers.

Happy landings!

(An edited version of this story was featured in VERTICAL Magazine’s Jun 2020 issue. You can access the story on page 56 of the magazine here)



©KP Sanjeev Kumar, 2020. All rights reserved. Cover photo by Kaypius.

Please consult national regulations, RFM and your company’s operations manual for rules as applicable to the type of operation you undertake. Views are personal and do not reflect those of my employer or the industry at large. I can be reached at kipsake1@gmail.com.

7 thoughts on “Understanding Helicopter Performance

  1. Kips very well explained the nuances of category
    A , B and performance 1 and 2
    Importance if calculating and checking the WAT charts for take off landings and more importantly the OEI drift down during crossing enroute altitude
    Have been trying to hammer the same during the Route checks and PCs
    Will keep it as a read to disseminate during training flights

  2. Thanks KPS. Another one raising very valid points to general lack of knowledge….. And in addition…… Willingness to understand the importance of planning for various conditions.

    And who is to blame….

    1. Are we doing it wrong at the basic training level…? No…. I dont think so….

    2. Even if we all flew simpler machines back then, what happened when we graduated to MEs….? Have things become so complex over the years that it passes up on common levels of understanding …..? No certainly not…..

    No clear answers evident except one trait that found common in my interactions in *commercial flying*.

    – to even acknowledge the seriousness it deserves

    – To learn and understand the required concepts

    – to put in the efforts of planning each and every single sortie we fly……

  3. Read this article now. Eye opener for some may be transitioning from single engine background to Multis.

  4. A good alround perspective of Helicopter operations. Because helicopter can be landed on a small patch of land it creates a false sense of security and the most deadly aspect is excursion from VMC to IMC based on individual judgement. More such successful excursions more is the pilot considered competent. It used to happen in commercial flying many decades ago. Today the minimum decides success of an approach not individual competence. Also in VIP and rescue missions the reluctance to call off is responsible for many accidents. So the success and failure of a mission and professional competence needs to be redefined.

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