Aug 11, 2003 was a black day in the history of Indian offshore industry. A Mi-172 helicopter (VT-MAF) chartered by state-owned Oil and Natural Gas Commission (ONGC) from private operator Mesco Airlines was on an offshore crew change sortie ex-Juhu helibase. Soon after liftoff from ONGC drillship Sagar Kiran with 25 passengers and 4 crew, the helicopter crashed into the sea with 27 fatalities. The accident report is not available in open domain, but issues with adjustment of rudder controls and “improper takeoff technique” finds mention in the ASN entry for this accident:
“Loss of Directional control and lift immediately after take off leading to helicopter hitting the sea water and crashing into sea. The combined effect of maladjusted rudder controls and improper technique used for take off from the helideck in the prevailing wind conditions led to the loss of the directional control and lift after take off.”
About eight years later, on Apr 19, 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, toppled and caught fire leading to loss of 19 lives. The investigation ruled out any mechanical failure or loss of power. The helicopter was simply overloaded while operating a contract that required Category A certified helicopter.
Lessons learnt at big cost of lives
The fatalities from these two Mi-172 accidents (46) account for the highest from any two civil helicopter accidents in India. It would be reasonable to assume that lessons from these accidents would have filtered down to the Indian armed forces, particularly 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 extracted a heavy price in lives, 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 9072 kg (20000 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 9072 kg (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 ‘no guaranteed stay-up capability’ after an engine failure. Another notable difference between these two categories is that Cat B references the HV diagram to shape their takeoff profile whereas for a Cat A helicopter operating under PC1 schedules, the HV diagram does not apply.
Gaps in civil-military performance certification
The foregoing is however applicable only in civil certification for the most part. Military follows certification standards (DEFSTAN, Milspecs, etc) that may be above (or below) 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, Cat B, etc are not common in the military pilot’s lexicon.
Another gap: As per 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, HV diagram for twin/multi-engine military rotorcraft are typically provided only for 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 world over have not adopted this standard. Often such gaps in certification and understanding of performance shows up in the “reject or continue” conundrum that faces military pilots executing a takeoff or landing.
The ‘reject or continue’ conundrum
Consider this example: You are taking off heavy from a short field. Your all up weight (AUW) is within stipulated weight, altitude & temperature (WAT) limitations specified in SOP. At some point on the takeoff segment, one engine fails. What would you do as a military pilot flying a multirole helicopter?
The answer would depend on two broad categories of rotorcraft — single engine and twin/multi-engine. For a single-engine helicopter (PC3), power loss at any stage of flight necessitates a ‘forced landing’. Not so for a twin engine rotorcraft. Depending upon the point of one engine inoperative (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 known as takeoff decision point (TDP) or landing decision point (LDP) while operating PC1. While operating PC2, the corresponding nomenclature is ‘defined point after takeoff’ (DPATO) and ‘defined point before landing’ (DPBL).
While exploring this topic, the answers to ‘reject or continue’ question I posed to military helicopter pilots ranged from “gee wiz, i never thought about this” to “TDP is knee speed”, or even a sketchy “depends whether I can maintain rotor RPM or not”. In the odd case, a better informed pilot may say “TDP is VTOSS”
Case for a deeper inquiry?
In the absence of published data in military rotorcraft flight manuals, one cannot fault the fuzzy understanding or the range of replies. In actuality, TDP or LDP will be defined as a point in space with height, speed, lateral distance from the liftoff or 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 OEM, attempts to emulate a Cat A takeoff or landing can be misleading, given that conditions specified for Cat A takeoff and landing include, among others:
- Minimum dimensions of runway, ground level helipad or elevated deck
- Wind limitations
- Obstacle limits in Final Approach and Takeoff area (FATO)
- Regulated takeoff and landing weights
- Selectable higher rotor RPM for takeoff and landing, if any
- Takeoff safety speed (VTOSS) and speed for best rate of climb (Vy)
- Sight picture (how the helipad should appear to the pilot on controls)
- Minimum height for elevated decks
- Dropdown height to ensure minimum 15 feet vertical clearance from all obstacles
- Usage of OEI contingency power or ‘target torque’
- Ceiling for Cat A operations
A sober reminder: all of the above did not prevent the 2011 Tawang crash where helicopter was overloaded and approach was non-conformal to contracted Cat A, PC1 schedules, if any existed. There can be no guarantees for negligence or deviation from published procedures.
Cat A certification and military flying
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. There could be many reasons for this, some of which, alongwith the likely outcomes, are highlighted below:
- Indifference to civil certification standards, including Cat A operations
- Gaps in understanding of performance classes across the civil-military divide
- Differences in safety paradigm (civil is highly focussed on pax safety; military on mission)
- Operating environment that either preclude Cat A operations in military or enforce unviable penalties
- Military staff requirements that do not expressly seek Cat A certification
- Varying specifications for helipads and elevated decks (HOSTAC, BRd766 for navy; CAP437 for civil)
- Approach / landing profiles and callouts that do not cater for safe ‘reject or continue’ in the event of OEI
- Risk ‘exposure’ during takeoff, landing and manoeuvres close to ground
- In the extreme, an unsafe contact with ground or water where a safe getaway could have been possible
The civil-military divide
On any given day in Mumbai offshore basin (erstwhile Bombay High), at least three different types of helicopters fly to oil fields in India’s Offshore Development Area (ODA). Shuttling between mainland VFR airfield Juhu (VAJJ) and offshore destinations, these helicopters deliver crucial men & material, keeping wheels of the economy flush with a very critical lubricant — oil. Bell 412 EP, Airbus AS365N3 Dauphin and Leonardo’s AW139/169 that operate within the ODA are all Category A certified helicopters contractually bound to operate under PC1 for all offshore takeoffs and landings.
A few miles south of Juhu lies Indian Navy’s helibase INS Shikra where an assortment of naval helicopters take to the skies each day for training flights, rescue missions, embarkations at sea, etc. Between INS Shikra and Juhu lies one of India’s busiest airports — Chhatrapati Shivaji International Airport (VABB). Co-located with CSI airport is an IAF helicopter unit that operates Mi-17 1Vs from Santacruz. CSI airport operates between 800-1000 civil flights daily — a record of sorts for a single runway. Every takeoff and landing on the 11000-feet main runway (09-27) by airliners from across the globe is executed strictly as per OEM and company manuals to ensure safety in case of an OEI event during takeoff or landing — meaning guaranteed safe reject or continue based on a critical decision point (CDP).
The interaction between pilots from across this civil-military divide is minimal — usually limited to pleasantries exchanged on R/T while transiting through each other’s airspace. The osmosis of knowledge and experience too is minimal, though military veterans occupy almost all civil helicopter cockpits in India. In short, the twain operate in silos.
Contrast in type of operations
The contrast in type of operations either side of the civil-military divide within this 10-mile circle is stark. 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 Performance Class 2. 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.
A few questions thus arise: Is Cat A certification or performance classes only applicable to civil-certified helicopters? Is the military resigned to operate Cat B in the absence of such information or is it by choice? Is there adequate awareness or sensitisation to category of rotorcraft and performance classes among military helicopter pilots? Finally, is there a need for military operators to also seek Cat A certification and appropriate schedules in their Rotorcraft Flight Manuals (RFM)?
Is Cat A for military an overreach?
Civil aviation is a highly regulated sector where passenger safety is accorded utmost importance. Any accident may not only lead to needless loss of life, but also erode passenger confidence in the industry – both totally unacceptable. Military, however, operates under a different paradigm of ‘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. For instance, how many military pilots will be able to explain or demonstrate a Cat A takeoff from runway, helipad and elevated deck for the type they fly?
The answer would be evident from military flight publications and pre-takeoff briefs that exclude terms such as takeoff decision point (TDP), landing decision point (LDP), takeoff safety speed (VTOSS), balked landing safety speed (VBLSS), rotation speed (Vrot), drop down height, etc. To be fair, this is not a comment on the military crew or how they fly. Cat A regulations include certification, flight profiles, limitations, normal / abnormal procedures and performance data that may altogether be missing 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 or considers this an overreach.
Applicability to transport category / utility helicopters
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 Independence. Underpowered Mi-8s were used as VVIP transport for the longest time; voids in performance were balanced by apex flying skills. The Delhi-based Communication Squadron (Comm Sqn) graduated to Mi-17s in due course, again without an explicit Cat A certificate. Finally, a capable VVIP transport — AW101 — was selected, only to be grounded and mothballed after the Agusta Westland scam surfaced and the company was banned. To this day, souped-up Mi-17 V5s without Cat A certification continue to fly the Prime Minister, President and VVIPs as we approach Azadi ka Amrut Mahotsav (75 years of Independence). I hope the “reject or continue” question piques the interest of Comm Sqn pilots.
Going forward, it may perhaps be prudent to look at Cat A certification while drawing up specs for transport / utility helicopters and invoke it as an option when needed. In multirole helicopters and those that are not customised for passenger/troop carriage, awareness of Cat A procedures will help the military pilot respond to the ‘reject or continue’ conundrum better.
To provoke a healthy debate on this issue, allow me to introduce a simple concept borrowed from the civil Cat A pilot’s lexicon.
The Magic of Takeoff Safety Speed (VTOSS)
Even keeping within WAT limits, the type of helicopter / takeoff landing profile you fly decides whether you land 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, DPBL, etc.
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 (“bucket” speed or Vy, best rate of climb 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 mention this speed in the rotorcraft flight manual (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 on approach, the initial segment of climb — typically upto 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 Vy in this segment will impinge on ‘height loss’ or ‘drop down height’, thus bringing the helicopter to an unsafe clearance from ground, water or proximate obstacles.
The AW139 flight crew operating manual (FCOM) defines VTOSS as “the airspeed at which the continued takeoff or Balked Landing Safety Speed scheduled climb gradient OEI (VBLSS) can be achieved“. It is 40 KIAS for elevated deck and short field on the AW139 and 50 KIAS for clear-area takeoffs. It cannot be far different for other helicopters. Velocity for best rate of climb ‘Vy’ is specified as 80 KIAS upto pressure altitude of 10,000 feet and 70 knots beyond that.
The Bell 412 EP manual on the other hand defines VTOSS as ‘airspeed that will assure required climb performances with one engine inoperative during Segment 2 of Category A procedures.’ In the Cat A supplement to Bell 412 RFM, VTOSS and Vy remain 40 KIAS and 70 KIAS for all Cat A profiles.
In general, VTOSS can be understood as ‘the minimum speed at which climb shall be achieved with the critical engine inoperative, the remaining engine(s) operating within approved operating limits’.
‘Required climb performance’
The significance of VTOSS must be understood within the context 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, including the ground or water below. A greater climb rate will of course be achievable at Vy; but in building up to that speed crucial 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. During the initial OEI climbout, primary focus of crew should be on maintaining correct attitude, target torque (OEI contingency power) and airspeed (VTOSS or Vy, as applicable to the segment), not operating switches or handles unless explicitly called-for in the checklist.
Please ponder over this extract from the Airbus A318/319/320/321 Flight Crew Techniques Manual:
“If an engine fails after V1, the flight crew must continue the takeoff. The essential and primary tasks are associated with the aircraft handling. The flight crew must stabilize the aircraft at the correct pitch and airspeed, and establish the aircraft on the correct flight path before the beginning of the ECAM procedure.
Performing the ECAM actions before the aircraft is stabilized on the flight path, reduces efficiency due to the PF’s high workload, and may lead to a trajectory error.”
(Note: For Airbus aircraft, V1 = decision speed; ECAM = Electronic Centralised Aircraft Monitor)
How it all comes together
Thus on a properly executed ‘clear area / runway’ Performance Class 1 takeoff (also called Cat A takeoff), the helicopter would have attained VTOSS by TDP (50 KIAS by TDP 30 feet for AW139). For elevated deck and ground level helipads, TDP is defined differently; sometimes 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 ‘path’, with height and speed together forming ‘gates’.
In a three-segment OEI climbout for elevated deck, for instance, segment 1 would include the rotation at OEI to build up to VTOSS; segment 2 would entail VTOSS-OEI contingency rating climbout till 200 feet above takeoff surface (ATS); segment 3 would involve level acceleration from VTOSS to Vy and Vy-max continuous power climb to 1000 feet above takeoff surface. The profile for ground level helipads and short fields may be different but the common factor remains VTOSS.
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 manoeuvres
The foregoing discussion was pertinent to twin/multi-engine helicopters. Single-engine helicopters always operate Performance Class 3 (PC3) which means a force landing in case of engine failure throughout the envelope. So where is the applicability of VTOSS, one may ask. Recall the procedure for ‘steep takeoff’ on the single-engine type you flew where one had to clear an obstruction on takeoff path (such as a restricted helipad)? Well, you may have not have known it as VTOSS, but the speed (approx 35-40 KIAS for an Alouette) that you rotated-to for this manoeuvre 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 universe, 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 (or best climb gradient speed, if VTOSS is not mentioned) till 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 up in all kinds of trouble in any power-limited situation.
Next time you are near a civil heliport operating Cat A helicopters under PC1, observe their takeoff profile. Notice how the crew accelerate quickly to a speed at which the power demand is low enough that a climb of 100 fpm can be maintained if one engine failed — keeping below 35 feet or so — followed by what looks like a normal takeoff, accelerating and climbing. The SOP would likely involve a pre-takeoff brief clearly enunciating if the takeoff would be Cat A or Cat B and the OEI actions as per RFM. Indicative “callouts” from pilot flying (PF) and pilot monitoring (PM) after “steady hover, power check” would read as below:
PF: “Initiating” (applies power, commences takeoff)
PM: “Power set”
PM: “Airspeed alive….VTOSS…positive climb”
PF: “TDP, committed”
At all times during this manoeuvre, both crew are in sync, ready to respond to ‘reject or continue’ in the event of an OEI. Now look into the SOP you follow as a military pilot and seek to answer the same question. Why end up in the drink when you can safely fly away? Even if you cannot, why not be aware? With awareness comes preparation and a better chance for safe outcome. Hope you never have to face such an event, but in my view good pilots should never rotate from hover without being able to answer “reject or continue”.
Happy landings, folks!
(An edited version of this article was first published by VERTICAL Magazine, Aug-Sep 2022 edition. You can access it here)
©KP Sanjeev Kumar, 2022. All rights reserved. Views are personal. Please follow approved RFM, SOP and company Ops Manual for the type/make of rotorcraft you fly.
About the Author
Kaypius is a former naval aviator and experimental test pilot with 4400 flying hours over 25 types. He is dual ATP-rated on Bell 412 & AW139 helicopters and a synthetic flight instructor on ALH Dhruv. Over 100 of his articles have been published in national and international magazines, journals and news media. He is ‘full-time aviator, part-time writer’ and blogs at https://kaypius.com/. He can be reached at firstname.lastname@example.org or on Twitter @realkaypius. Views are personal.