features Reject or Continue? Understanding Category A Performance

We look at the military/civil divide over Category A certification, and why military pilots could benefit from a greater understanding of it when it comes to power failure during takeoff and landing.
Avatar for KP Sanjeev Kumar By KP Sanjeev Kumar | August 29, 2022

Estimated reading time 13 minutes, 36 seconds.

Aug. 11, 2003, was a black day in the history of the Indian offshore industry. An Mi-172 helicopter chartered by the state-owned Oil and Natural Gas Commission (ONGC) from private operator Mesco Airlines was on an offshore crew change flight from Juhu helibase. Soon after liftoff from the ONGC drillship Sagar Kiran, with 25 passengers and four crew onboard, the helicopter crashed into the sea. Twenty-seven of those on board died. The accident report is not available in the open domain, but issues with adjustment of rudder controls and “improper takeoff technique” find mention in the Aviation Safety Network entry for the accident:
“The combined effect of maladjusted rudder controls and improper technique used for takeoff from the helideck in the prevailing wind conditions led to the loss of the directional control and lift after takeoff.”

An Indian Air Force Mil Mi-17 V5 prepares for a flight.

About eight years later, on Apr 19, 2011, Pawan Hans Ltd. was operating an Mi-172 helicopter on the Guwahati-Tawang sector in northeast India. While on short final approach to the high-altitude Tawang helipad, the helicopter, with four crewmembers and 19 passengers, descended below the level of helipad. It impacted with the concrete edge, toppled, and caught fire. Nineteen of those on board lost their lives. The investigation ruled out any mechanical failure or loss of power. The helicopter was simply overloaded while operating a contract that required a Category A certified helicopter.

The 46 fatalities from these two Mi-172 accidents account for the highest from any two civil helicopter accidents in India. It would be reasonable to assume that lessons from these accidents filtered down to the Indian armed forces, and particularly the Indian Air Force (IAF) — the single largest operator of Mi-series helicopters in India.

An important change happened between these two accidents. After the 2003 accident, all offshore helicopter operations in India were upgraded to Performance Class 1 (PC1). As per EASA CS-29.1 (c): “Rotorcraft with a maximum weight greater than 9,072 kilograms (20,000 pounds) and 10 or more passenger seats must be type certificated as Category A rotorcraft.”

This covered large helicopters like the Mi-172 in civil domain.

CS-29.1 (f) specifies that: “Rotorcraft with a maximum weight of 9,072 kilograms (20,000 pounds) or less and nine or less passenger seats may be type certificated as Category B rotorcraft.”

In general, Category A provides for critical engine failure performance capability to achieve either a safe reject or continue. Any rotorcraft that is not certificated Cat. A will fall under Category B — simply meaning it has no guaranteed stay-up capability after an engine failure. Another notable difference between these two categories is that Cat. B aircraft reference the height-velocity (HV) diagram to shape their takeoff profile, whereas for a Cat. A helicopter operating under PC1 schedules, the HV diagram does not apply.

This is, however, applicable only in civil certification for the most part. The military follows certification standards (Def Stan, MilSpecs, etc.) that may differ from corresponding civil standards. Military helicopters routinely operate to helipads and landing areas that meet the definition of “congested hostile environment” — yet terms like PC1, PC2, Cat. A, and Cat. B are not common in the military pilot’s lexicon.

Another gap: As per EASA CS-29.87 (b), “for single-engine or multi-engine rotorcraft that do not meet the Category A engine isolation requirements, the height-velocity envelope for complete power failure must be established.” While this is followed for single-engine helicopters in military service, the HV diagram for twin/multi-engine military rotorcraft are typically provided only for one engine inoperative (OEI) and not complete power loss.

Today, most modern, twin-engine helicopters in the civil segment — even light twins — are certified Cat. A by manufacturers. However, militaries the world over have not adopted this practice. Such gaps in certification and understanding of performance show up in the “reject or continue” conundrum that face military pilots executing a takeoff or landing.

Reject or Continue?

Consider this example: You are taking off heavy from a short field. Your all-up weight (AUW) is within stipulated weight, altitude and temperature (WAT) limitations. At some point on the takeoff segment, one engine fails. As a military pilot flying a multirole helicopter, what would you do?
The answer would likely depend upon two broad categories of rotorcraft — single-engine and twin/multi-engine. For a single-engine helicopter (Performance Class 3), power loss at any stage of flight necessitates a “forced landing.” Not so for a twin-engine rotorcraft. Depending upon the point of OEI, one of two options may be available: reject takeoff (RTO) or continue takeoff (CTO). The identification of a “decision point” thus becomes crucial. Such a decision point is clearly defined in terms of height/speed, and is known as takeoff decision point (TDP) or landing decision point (LDP) while operating PC1. While operating in PC2, the corresponding nomenclature is defined point after takeoff (DPATO) and defined point before landing (DPBL).

Crash investigators explore the wreckage of a Mil Mi-172 during accident analysis. Accident Investigation Bureau Photo

TDP or LDP will be defined as a point in space with height, speed, lateral distance from the touchdown zone, or a combination of these. It is applicable only while flying a specified profile within regulated Cat. A schedules. Without such data or certification by an OEM, attempts to emulate a Cat. A takeoff or landing can be misleading.

Despite a slew of changes mandated by regulations after fatal crashes related to helicopter performance and certification standards, militaries across the globe are indifferent to Cat. A certification even if an equivalent civil variant flies in the civil segment.

The Civil-Military divide

On any given day at least four different types of helicopters fly from Juhu helibase to oil fields in the Mumbai offshore basin. The Bell 412 EP, Airbus AS365 N3 Dauphin and Leonardo’s AW139 and AW169 are all Cat. A-certified helicopters, and as such, are bound to operate under PC1 for all offshore takeoffs and landings.

A few miles south of Juhu lies the Indian Navy’s helibase INS Shikra, where an assortment of naval helicopters take to the skies each day for training flights, rescue missions, and embarkations at sea. While all offshore civil helicopters operate Cat A, military helicopters take to the skies ticking “VFR” and “special military operations” boxes in their flight plan. Unbeknownst to most military crew, their helicopter would fall under the ambit of Cat. B certification: takeoffs and landings are typically PC2. Even if an equivalent civil-certified variant exists for the type they fly, the required schedules, performance data and profiles are conspicuous by their absence in most military variant flight publications.

Civil aviation is a highly regulated sector where passenger safety is accorded utmost importance. The military, however, operates under a slightly different paradigm where it is “mission first; safety always.” While flying off ships in the navy, we did what we could within the framework of operational exigencies to ensure safety. Indeed, there was a time when best practices flowed from the military to civil aviation. One wonders if this holds true today.

This is not a comment on military crews or how they fly. Cat. A regulations include certification, flight profiles, limitations, normal/abnormal procedures and performance data that may be missing altogether from a military flight manual simply because the military user never asked for it. In a competitive and price-sensitive industry, no OEM can be expected to seek Cat. A certification if the military customer itself is indifferent to it.

However, military flying often includes carriage of passengers over “hostile” terrain. Military helicopters have been used to transport VVIPs and heads of states in India since the country achieved independence. To this day, souped-up Mi-17 V5s without Cat. A certification continue to fly the Indian prime minister, president and heads of state.

Going forward, it would perhaps be prudent to look at Cat. A certification while drawing up specs for utility helicopters, or use it as an option to be exercised at the captain’s discretion. In multirole helicopters and those that are not customised for passenger/troop carriage, awareness of Cat. A procedures will help military pilots respond to the “reject or continue” conundrum better.

The Magic of Takeoff Safety Speed (VTOSS)

Even keeping within WAT limits, the type of helicopter/takeoff landing profile you fly decides whether you end up in the drink, on deck, or make a safe diversion in the event of losing a critical power unit. While operating from ships, this could mean an unplanned ditching or a spectacular “arrival” on a small deck with uncontained momentum — both hazardous. Yet, during my entire career as a naval aviator, I was either ignorant or couldn’t care less for terms like TDP, LDP, DPATO, and DPBL.

Helicopter pilots, especially those who have only flown single-engine helicopters, would likely have missed a speed known as VTOSS (takeoff safety speed). It lies in the region between hover and minimum power speed. We transition through it briefly during approach and landing; it may be close to the speed obtained at knee-point of a height-velocity diagram — but is defined and notified separately in the RFM of a Cat. A helicopter. Cat. B and single-engine helicopters may not contain this speed in the RFM.

But the importance of VTOSS cannot be understated. On this speed pivots the chances of a safe recovery from any OEI or power-limited situation. In case the pilot elects to continue takeoff or undertake balked landing after an OEI event, the initial segment of climb (typically up to 200 feet above takeoff surface) has to be conducted at VTOSS so that the best climb gradient is obtained. This may involve a rotation to VTOSS. Any attempt to build-up speed to best rate of climb (Vy) in this segment will impinge on the drop down height, thus bringing the helicopter to an unsafe clearance from the ground, water or nearby obstacles.

In general, VTOSS is defined as the minimum speed at which climb can be achieved with the critical engine inoperative and the remaining engines operating within approved limits.

The significance of VTOSS cannot be understood without knowing the meaning of “required climb performance.” For civil certification, this means the ability to maintain a minimum climb rate of 100 feet per minute. But more important than climb rate, VTOSS defines the ability to maintain a climb gradient that will allow the helicopter to fly clear of obstructions. A greater climb rate will of course be achievable at Vy, but in building up to that speed, height will be lost, leading to a less-than-safe climbout on single engine. Note that the numerator in both climb rate and climb gradient is the same — height. However, the denominator is different — “unit time” for Vy and “unit distance” for VTOSS. This distinction is crucial in the obstacle-ridden environment that a twin/multi-engine helicopter suffering an OEI must navigate safely in the initial climbout.

A Pawan Hans Ltd. Airbus AS365 N3 + flies off the coast of India. As a Category A-certified helicopter, it is bound to operate under PCI for all offshore takeoffs and landings. Anthony Pecchi Photo

Thus, on a properly executed “clear area/runway” PC1 takeoff, the helicopter would have attained VTOSS by TDP. For elevated deck and ground-level helipads, TDP is defined differently; mostly in terms of height and a “sight picture.” But in case of OEI after TDP, the first and foremost action would be to rotate or build up to VTOSS, even as the crew attempt to regain lost rotor RPM. The CTO is thus defined in “segments” or “paths,” with specified height and speed — together forming “gates” — becoming critical factors.

The same speed while on landing approach is termed VBLSS. Depending on the type of landing surface and the point of OEI, VBLSS will determine the outcome of a balked landing and safe climbout to return/divert for a safe OEI landing.

Applying VTOSS to other maneuvers

The foregoing discussion was pertinent to twin/multi-engine helicopters. Single-engine helicopters always operate Performance Class 3, which means a forced landing in case of engine failure throughout the envelope. So, where is the applicability of VTOSS? Recall the procedure for “steep takeoff” on the single-engine type you flew where one had to clear an obstruction on takeoff path? Well, you may have not have known it as VTOSS, but the speed that you rotated to for this maneuver was the speed that gave you maximum height per unit distance — crucial to clear that treeline or building on takeoff path! Strangely, in the military helicopter pilot’s world, range speed, loiter speed, dash speed, etc. are overrated — to the utter disregard of VTOSS!
It doesn’t end with takeoff or Cat. A operations alone. Even if you are making a power-limited approach or executing a running landing due to tail rotor direction control failure, or any other power-limited situation — on final approach, delay that deceleration below VTOSS until you are sure of making the landing area. Below this speed, the region of reverse command sharpens — an area where the power requirement sharply rises as you go slower and slower. Entering this zone with inappropriate height can land you in all kinds of trouble in any power-limited situation.

As a military pilot, there is no reason why you shouldn’t be aware of the performance class you are operating in. With awareness comes preparation, selection of an appropriate takeoff/landing profile, and thus better chances for a safe outcome. Hopefully, you never have to face an OEI event, but in my view, good pilots should never rotate from hover without being able to answer “reject or continue?”

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  1. Avatar for KP Sanjeev Kumar
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2 Comments

  1. Very informative article and a must read for all helicopter pilots.

  2. An erudite article by a professional helo pilot. I agree with the views of the author. It is very important in safe operations in both civil and military helicopter operations.
    My German instructor on the EC-135 during the early 2000s advised me to stick to the Cat A profile as far as possible, whether operating PC1 or PC2. Although I have followed his advice, I have luckily never faced an OEI to know about the wisdom of his words.
    Military helicopter aviation may also require them to operate beyond the Cat A altitude limitations of a particular type, going by the ‘Mission first’ formula required to be followed there. I feel that the PC1 requirement is usually customer-driven prerogative in ‘civvies street’ wherein he chooses to opt for the affordable safety standards, since the economic viability of operating the helicopter comes in. However, safety of life and limb would be paramount for any passenger.

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