It’s a couple of hours after the sun set into the ink-blue ocean. Ashore, people are returning home, cursing the traffic & responding to WhatsApp messages. Dinner is cooking in many kitchens. If you hurry, you may still be able to catch a warm shower and the 9 O’Clock news.
No such luxuries for you, naval aviator! From the cockpit of a modern naval multi role helicopter (NMRH), you have seen the last vestige of daylight disappearing with the horizon a few hours ago as you return for landing on ‘mother’ after another mission. Destination reports a 30 knot crosswind and partly cloudy skies on a clear moonless night.
There is no VOR/ILS to help you, no aerodrome beacon, no autonomous approach or landing aids. Fuel is down to minimum. There is no diversion, nowhere else to land. And if that’s not enough, the landing pad is no bigger than a squash court and moving about six axes.
Big Challenges, Unsung Heroes
It’s a task that has challenged the best helicopter pilots since middle of the last century and continues to do so. Each day, each night, unsung heroes operate from naval ships in challenging conditions to execute highly complex oversea missions.
The rotary wing element of embarked naval aviation forms a very vital punch in all navies of the world. The helicopter’s inherent ability to operate from small, confined helidecks and stage-through afloat platforms, has made them the eyes, ears and ‘long arm’ of the ships. Some of the traditional roles undertaken by helicopters in navies around the world are search and rescue (SAR), logistic support, casualty evacuation and communications link. To these have been added specialized roles such as Anti-Submarine Warfare (ASW), Anti Surface Vessel (ASV), Electronic Warfare (EW) and Airborne Early Warning (AEW).
In the new millennium, nature of battle itself has transformed due to the evolving security landscape, maritime terrorism, 9/11 etc. The focus of military activities has shifted as a result of changes to international security created by asymmetric threats from state and non-state actors. Battlefield dimensions have grown dramatically while timeframes have shrunk.
While nations grapple with these changes, the focus of maritime helicopter operations have also slowly shifted from conventional roles to tactical lift, land support operations, humanitarian & disaster relief, counter-piracy operations, maritime interdiction operations, environmental control (MARPOL) etc.
This expanded domain of operations requires newer and more ‘strap-on’ capabilities in both helicopters and ships while retaining some of the old world charm. Some of my colleagues like to remind me that UAVs (fixed wing & rotary wing) will soon make us helicopter pilots (and who knows – all pilots?) redundant and snatch our jobs. But far from becoming redundant, helicopter operation from naval ships, I argue, is here to stay. Changing situations only dig deeper into the helicopter’s intrinsic flexibility.
It is widely acknowledged that shipboard operations are among the most challenging of any piloting task. Operating from small decks with poor visual cues in low visibility coupled with the dynamics of airwake turbulence and ship motion is a challenge for any naval helicopter pilot. Added to this are the challenges of over-sea missions, the highly corrosive medium in which the navy operates and finally the management of classic ‘fleet air arm’ variables!
Presently, Indian Navy operates helicopters from a wide variety of ships varying in size from a small sized landing ship to majestic aircraft carriers. We have in our inventory helicopters and ships of indigenous, Western and Eastern origin. This brings about a plethora of challenges to the naval commanders due to stringent and unique requirements and limitations of ships and helicopters. The classes of ships capable of operating helicopters are as diverse as the types of helicopters themselves.
The nature of naval helicopter operations require the helicopter to quickly adapt to a host of seagoing platforms that vary in design, available facilities and capability. Integration of helicopters with a host ship therefore starts from design stage where minute details of both the ship and integral helicopters are iterated to give the best operational advantage. The integration process is followed up through regular interactions between experts on both sides and culminates in ground and flight trials for development of Ship Helicopter Operating Limits, or SHOL as is commonly known.
The additional limitations pertaining to operations from moving afloat platforms, or SHOL, is essentially a set of limiting conditions related to ship, helicopter, onboard systems, tie-downs and ambient conditions under which helicopter operation from ships are allowed to be undertaken. There is a tendency to refer to this as ‘wind envelope’ whereas SHOL actually encompasses more than just the wind envelope. The wide variety of helicopters in Indian inventory that include Western origin, Eastern origin and indigenous helicopters, with designs dating from the latest Kamov-31 AEW helicopter to some which are over 50 years old, throws up its own challenges.
(Picture courtesy Indian Navy, INAS 339, ‘Falcons’)
Helicopters are an integral part of the fleet air arm and that means we have to coexist and blend with the warship and air elements of the fleet at sea. Since the ship is home to different departments with varied capabilities, location and co-habitation requirements often throw up challenges. During the design phase of a new ship therefore, there is potential for many compromises as systems jostle for space and prime ship locations. These compromises often affect helicopter capability and availability. They have to be surmounted with ingenuity.
In most warship designs, the helicopter deck is usually a very confined area situated at low freeboard in the lee of ship’s exhaust and radiating aerials. Guns, radars, and their stabilisation & firing arcs take predominance in any designer’s calculus – followed as an afterthought by crew comfort. Alas, most ship designs are conventional; which means more challenges for us helicopter pilots as we always get the ‘wrong end of the ship’!
You also have to contend with ship motion. Ship movement in six degrees of freedom is a function of ship’s design features and oceanic parameters. The combined effect of ship and oceanic conditions define the ship’s dynamic behaviour while underway. Needless to state, these dynamic factors result in several potentially hazardous conditions on a deck. The picture below is Kaypius measuring the last few inches before the tail ‘strikes’ in a SHOL trial!
The various types of ship-helicopter integration trials that are usually undertaken range in complexity from experimental trials of new ship-new helicopter integration trials to revalidation trials after modifications or refit. These trials could involve upto 10-15 hrs for a basic helicopter such as the Allouette IIIB to 40-50 flying hours, or even more for a multi role helicopter.
Why the need for additional testing when the helicopter has already been put through the paces during design and certification by OEM?
Helicopter-ship operations have been going on since middle of the 20th century. Basic helicopter limitations are usually determined ashore by the aircraft manufacturer (usually in beautiful places like Florida, Marignane or Bangalore!) But naval helicopters are meant to deliver out at sea. Land-based aircraft limitations may therefore not be completely valid in the shipboard environment due to the individual and combined effects of some of the following factors:
- Ship airwake / turbulence
- Ship motion
- Confined landing area
- Visual cue limitations
- Sea spray and ship’s exhaust gas nuisance
During ship-helicopter integration trials, several variables are accounted for, singly and in combination with other factors to ensure that, as far as possible, all combinations are tested safe for the average fleet pilot.
But what typically happens is this – SHOL trials involve various variables like ship, aircraft and test crew availability, weather and sea state, flying effort, testing aids etc. Sometimes the biggest variable is the ship’s availability. All the test variables / conditions such as wind, sea state, temperature (and sometimes even the test crew!) etc cannot be controlled to our liking. From my experiences, the only parameter that can probably be changed independently is the test pilot (if we don’t like him)! In order to map the influence of all variables, SHOL tests in most cases have to be done in phases. This requires a staged approach, foreplanning and patience, and quite understandably, apart from some inherent ‘build issues’, the final capability achieved would depend on how much time, effort and resource was invested in the whole process.
So what are the ‘build’ issues…is it the helicopter …or is the ship to blame?
To be fair, ‘build’ issues that contribute to SHOL lie on both sides of the ship-helicopter divide. Actually, if we were to peg piloting skill to that of an average pilot, there are only two main factors – helicopter characteristics and ship characteristics.
Before the helicopter manufacturers start to frown at me, let me explain – there is nothing wrong with your helicopter. It’s just that while making the helicopter, the designer has to deal with several mutually exclusive, essential and often contradictory requirements. So it’s usual to ‘rob Peter and pay Paul’. Stability vs agility, hover performance vs forward flight performance, blade folding mechanism vs rotor type, hover ceiling vs dash speed, gross weight vs useful payload, sensor location versus ground clearance, shipboard compatibility etc, to name a few. Some of those factors that have been compromised or optimised may eventually impinge on SHOL. Assuming that power is not a major factor at sea level, some of the important aircraft related factors are:-
- Stability and handling characteristics of helicopter
- Permissible low speed envelope of the helicopter
- Recovery assist system available on the helicopter
- Type of landing gear
- Design of tail section (e.g straight / canted)
- Slope landing capability
A helicopter designed with better low speed characteristics will thus give you greater freedom in low speed or cross wind handling. Design features would determine how much of crosswind and tailwind the helicopter can withstand before control is lost or pilot workload becomes unacceptable. Highly agile helicopters such as the Advanced Light Helicopter, Lynx, Seahawk or the fly-by-wire NFH90 have very large low speed envelopes. These aircraft may therefore have better SHOLs as compared to vintage helicopters with restrictive low speed envelopes. It is quite usual for ‘ship drivers’ to find the turf on the other side greener, especially when they find helicopters from other navies launching during multilateral exercises at sea when their own cannot be allowed to fly because of restrictive SHOLs. The answer is simply camouflaged in some of the reasons I mentioned here.
Ship motion is another thing that we have to deal with all the time. My friends on the ships often go “Ah, ship motion! ~ you touched a raw nerve there”. Their legitimate lament is that some ships have such restrictions on roll and pitch as to practically rule out flying operations during heavy seas. Why is that?
So I explain to them that it is ultimately a function of how much testing has been done. Ship’s pitch & heave motion for example, though relatively slow and predictable, can result in large and rapid vertical movements of the flight deck. This motion could exceed the vertical agility of the operating helicopter, exceed the landing gear or engine torque limits, or increase the risk of a tail strike. Straight-tail helicopters like the Alouette, Sikorsky S-76 or Airbus Dauphin need special consideration here due to low tail clearance as compared to helicopters with tail rotor mounted on elevated pylons like the Seaking or ALH, which means we need to be more conservative with deck motion and obstructions in vicinity of the tail for such helicopters. But on the flip side, canted or high-tail helicopters usually bring with them additional ladders and maintenance platforms that are again not very welcome onboard ships. These are just some of the contradictions that naval aviators and OEMs providing helicopters to the navy have to contend with.
As far as the warship is concerned, proceeding from an aircraft carrier floating on a glassy calm sea, in some conditions, flying from a FFG/DDG in heavy seas could be too dangerous to even try. There are several ways in which ship design impinges on helicopter capability some of which are listed below:
- Layout of flight deck and disposition of landing spot
- Design of hangar and superstructure ahead of the flight deck
- Serviceability and efficiency of ship’s stabilizers
- Recovery assist system available on the ship such as the RAST, ASIST etc.
- Landing aids available on the ship (deck motion systems, wind measurement systems, visual landing aids etc)
- Disposition of main engine exhausts
- Maximum speeds
- Tiedown fittings, both on aircraft and deck
- Ship’s dynamics while underway
Since SHOL development is an expensive and time consuming activity, shouldn’t we explore alternate methods to control / predict the variables and generate SHOL analytically?
The answer is quite obviously YES. Yet, the ability to predict helicopter/ship interface envelopes analytically remains a challenge. The cost of testing and the inability to provide all test conditions has however made it a very attractive option for few nations. The UK and USA have strong programs to support future helicopter/ship qualification testing through computational fluid dynamics – CFD or wind tunnel experiments. The Netherlands have successfully used a combination of wind tunnel data of ship model, full-scale airwake data, and land-based helicopter data to predict operating limits prior to the actual helicopter/ship flight testing. But these are usually national programs with large funding & resources. Very few private entities (Prism Defence, Australia is one) provide consultancy & flight test services for ship-helicopter integration (see video here).
As far as alternate solutions are concerned, my experience is that small investments of time & specialist aviation oversight while designing a ship can bestow disproportionate benefits in a better helicopter deck to operate from. So, if you can afford it, please enlist SHOL specialists at design stage to improve interoperability & achieve full operational capability faster (or payout in millions for fixing ‘warranty period’ or ‘in-service’ glitches).
Yet, I am not aware of any program which is totally reliant on laboratory analyses or computer simulation and at the end of the day, SHOL has to be developed through actual flight tests (which thankfully keeps SHOL test pilots like me still in business!).
So what’s the recipe for future? If we look back, although technology has progressed in leaps and bounds, for most countries, that little piece of real estate at aft end of the ship known as the ‘helideck’ has somehow lagged behind the technological curve. In this age of smartphones and ‘apps’ for every conceivable activity, we may find legacy devices and aids such as inclinometers for ship motion, vane-type anemometers and incandescent lighting still used on some naval ships. Modern shipborne aids such as high-accuracy ultrasonic wind sensors, inertial ship motion systems, predictive landing period indicators, gyro stabilised Glide Path Indicators and horizon bars, NVG compliant visual landing suite etc should be the rule not exception, given the complexities a naval aviator has to anyway contend with.
Also, I see a HUGE scope for collaboration between stakeholders. Ships, helicopters & flight testing places heavy demands on individual companies’ and even a nation’s resources. Why duplicate efforts and fritter away flying hours & resources? Advances & collaborations in this field, coupled with technologies for a modern NMRH under development, should make the job of take-off & landing easier, thus leaving naval pilots with more than adequate spare capacity to concentrate on MISSION, always and every time.
Thank you. I welcome your views.
©KP Sanjeev Kumar, 2019. All rights reserved. I can be reached at firstname.lastname@example.org. Views are personal.
A buoyant extrovert, forever charmed by the wonderful world of flight and flying machines, Kaypius spent a quarter-century serving the Indian Navy (IN) where he flew helicopters ranging from humble Alouettes to the venerable Kamov-28s, ALH & all that IN & IAF threw at him. He is a qualified Experimental Test Pilot and DI professional from US Naval Test Pilots School (USNTPS). He holds a dual ATP rating on Bell 412 & AW 139 helicopters with over 6000 deck landings. He calls himself ‘full-time aviator, part-time writer’ and blogs at www.kaypius.com.
This article has been excerpted from a paper presented by him at Aero India International Seminar 2013. The paper was also presented in a technical seminar at IIT Delhi (2016), IIT Kanpur (2014) and published in aerospace journals. Feel free to share.