Recently, I launched a paper aeroplane from the edge of a cliff to the delight of my son. As we both watched it glide smoothly down to the valley below, my mind rewound back to the days when as test crew we cut the throttles on perfectly serviceable helicopters to study engine failure dynamics. Needless to say, I am much happier today launching paper planes down hillsides!
Helicopters are Different
Helicopters typically have a lot more moving parts than aeroplanes. Piston or turboshaft engines, gear trains, main rotor, tail rotor, lubrication pumps–all these rotating components are required to keep it in the air. Consequently, failure of any of these critical units can cause the helicopter to drop from the sky unless the pilot handles the situation skilfully. One of the foremost things taught to helicopter pilots is therefore the handling of a power loss situation brought about either due to an engine failure or situation requiring the engine to be deliberately shut down in flight.
Engine Failure Dynamics
Many things happen when you lose an engine on a helicopter. Much of the immediate actions required depends on the type of machine you were flying and the conditions at time of failure.
Consider these two scenarios while piloting a helicopter:
Case 1: You are hovering a light, single-engine H125 AStar in hot & high conditions. The prevalent combination of weight (mass, actually), altitude and temperature (WAT) puts you on the fringes of the helicopter’s performance. Suddenly, you experience a sharp yaw, loss of height and observe your engine and rotor RPM winding down rapidly. Oops, you just lost your only engine!
Case 2: You are cruising in a lightly-laden, twin-engine AW139, 2000 feet above mean sea level (MSL) on a ‘standard’ day. Power requirement is low and Power Index (torque in this case) is safely in the green zone. You are well clear of terrain and obstacles. Your instruments and CAS (Crew Alerting System) are looking nice & green. Suddenly, you experience a mild yaw, master warning lights up, ‘1 Eng Out’ illuminates and ‘bitching betty’ chimes in. Failed engine parameters wind down but the transient droop in Rotor RPM is hardly perceptible; so is the change in engine noise with the doors closed and air conditioner ON with fan speed ‘high’. Oops, you just lost one of two engines!
The two situations described above indicates the range of conditions under which an engine may quit. The reaction time and criticality also vary accordingly.
With continuous improvement in aerospace technology and high reliability of aero engines, engine failure on helicopters has become extremely rare. However, as flight crew we always have to be aware and alert to this exigency. Knowledge of this condition and recognition of symptoms in time will prepare us for a safe transition to either one engine inoperative (OEI) flight in a twin-engine helicopter or autorotation in a single engine helicopter.
Let us start by examining a power loss condition on a single engine helicopter.
Single Engine Helicopters
When the engine quits on a single engine helicopter, the power required to drive the rotor must come from somewhere else. If you have recognised the symptoms in time and lowered collective, the descent airflow will change the blades’ angle of attack, thereby generating an autorotative area on the disc that provides the power required to keep the rotors going. If descent is not enabled in time, the helicopter will start feeding on its own rotor energy thus slowing down the rotor. But there is a finite delay plus height loss involved in any recovery action due to following reasons:
- Pilot reaction time to recognise the condition and start lowering collective (A)
- Time involved in lowering the collective lever (B)
- Transition time in settling into autorotative descent by which time rotor RPM will reach a minimum point (C)
- Time required to reverse the initial drop in rotor RPM to reach steady state autorotative RPM (D)
The Energy Exchange
The interesting part in this calculus is that power required comes from height lost as potential energy is exchanged for kinetic energy! Everything happens with loss of either rotor RPM or height. How low the rotor RPM droops would therefore be a function of all of the above factors. How soon the slowing down occurs depends on WAT conditions and inertia of the rotor system. Tip weights are thus often added to main rotor blades to improve rotor inertia.
‘A’ denotes the ‘lever delay time’ that is involved in any recovery action. There is a certain ‘A’ which will make the rotor RPM reach minimum permissible power-off value for that condition. This time delay is known as the ‘critical lever delay time’. If you hold on to the collective beyond this time, the rotor RPM will droop below minimum safe, coning angle will increase sharply, lift will be lost and recovery could well be rendered impossible below this ‘divergence RPM’. The blades have stalled on your helicopter one last time.
Lever Delay Time and the H-V Diagram
The critical lever delay time is not a constant and depends on the rotor RPM, condition of flight and power setting at the time of failure. OGE hover or vertical climb under ‘hot & high’ conditions with a low-inertia rotor system can produce very small delay times (less than a second in some cases). Test crew run these tests on prototype helicopters to define combinations of height and velocity which need to be avoided in order to ensure adequate recovery margin for safe recovery from engine failure. The results of several mathematical and aerodynamic calculations and flight tests define the H-V diagram, also known as the ‘Dead Man’s Curve’ for good reason.
Now consider a single-engine Cheetah (Lama) helicopter taking off from a table-top helipad at 23000 feet density altitude in the Siachen Glacier and you will know why I have the greatest respect for pilots who undertake these tasks.
The basic theory of engine failure doesn’t change. However, power sharing between the engines on a twin or multi-engine helicopter caters for OEI or partial power conditions by compensating with the live engine to the extent it can. Engine governing systems in modern twin-engine helicopters are designed such that engine power scheduling is intentionally set to a ‘higher than required’ power level and trimmed down so as to ensure failure protection to a higher power setting. In the event of OEI therefore, the live engine can increase power up to its OEI contingency rating (usually a 2.5-min rating).
Depending on the condition of flight, rotor RPM at point of failure may droop transiently or settle down at the lower limit of safe governed NF/NR defined for OEI flight. If the ‘Low Rotor RPM’ warning is triggered, it is indicative of a power deficit developing either because of WAT conditions or mishandling. Don’t ignore this warning.
Some Rotorcraft Flight Manuals like that of the Bell 412EP specify a ‘Target Torque’ which is the OEI maximum contingency rating on the live engine (a 2.5-min rating on the Bell 412EP). This is ‘live engine torque’ and not ‘mast torque’. Knowing the ‘target torque’ for the WAT conditions at take-off or landing helps the pilot understand how much ‘delta’ is available before you hit a power limit. Exercise caution when operating All Engines Operative (AEO) at mast torque close to single-engine ‘target torque’, especially at low heights. During AEO take-off or landing, if your mast torque is above this figure, only a positive reduction in collective will prevent excessive rotor droop or ‘over-torquing’ of the live engine (although, strictly speaking, torque is not an engine performance limit). This may come at a significant loss of height, particularly important while operating over hostile terrain or offshore.
Companies and regulators usually frown upon pilots who bring back helicopters safely with one failed engine and another engine whose limits have been blown. This is sad but one should not hesitate to use the contingency rating when required. Knowledge of the ‘delta’ and smooth, conscious control manipulation may hold the key to a successful transition. The Indian ALH (Dhruv) helicopter with the Turbomeca TM333 2B2 engine, for instance, has a 30-second ‘Super Contingency Rating’ (SCR) that is available to the pilot but if selected and used, the engine will have to be removed from the helicopter and returned to the factory. Too bad but that shouldn’t weigh you down when you need to use that power to save the helicopter and its occupants.
In the rare event of a double engine failure (usually due to fuel starvation or mishandling) on a twin-engine helicopter, we are now faced with a situation that brooks no delay in recovery action. Terms like critical lever delay time, divergence RPM etc discussed in parlance of light singles should ring aloud in your mind. It’s time to use your autorotation skills again, this time with a much heavier bird!
H-V diagrams provided for twin-engine helicopters are applicable for an OEI condition and not total power loss (total power loss on a twin-engine helicopter is too risky to be tested without incurring damage). To borrow a line from the AW139 RFM, the H-V diagram defines, in the event of a single engine failure during take-off, landing or other operation near the surface, a combination of airspeed and height above ground from which a safe single engine landing on a smooth, level and hard surface cannot be assured. Note the terms ‘smooth, level and hard surface’. Also remember that for the H-V diagram to be valid, you have to be within the WAT limits specified for your helicopter.
Increasing Rotor RPM for take-off and landing
Some twin engine helicopters incorporate a power turbine governor actuator switch to ‘beep’ up the free turbine (NF/N2) & (thus) rotor RPM to a value which is 2-4% more than the nominal governed RPM of 100% for take-off and landing. The Bell 412EP incorporates a INCR-DECR beep switch which can trim the rotor RPM from 97% to 101.5% while the AW 139 and A109 helicopters have a two-way switch which can set the rotor RPM to either 100% or 102%. For all Cat A take-offs and landings, 102% RPM is to be selected on the AW139. This possibly stems from Category A certification requirements that specify a certain reaction time before initiating recovery action (typically delay time is about 1-2 seconds) in the event of OEI.
Benefits of Increased Rotor RPM
In my analysis, this function is provided to maximise your chances for a safe landing or getaway in the event of an OEI and not to improve AEO performance. That said, higher rotor RPM for take-off and landing accrues the following benefits:
- Power is directly proportional to product of torque and rotor RPM. Thus higher rotor RPM means lower torque for the same power requirement with all engines operating. Or more power for the same torque, whichever way you like to see it.
- With a higher rotor RPM, the angle of attack and thus blade pitch for the same power is slightly less. At high angles of attack, this improves the lift-drag ratio of the blades.
- More RPM means more energy stored in the rotor – always a good thing if you lose power!
- In the event of losing an engine, the higher rotor RPM gives greater reaction time to pilots before lower limit of rotor RPM is encountered. Going from our theory for single engine helicopter, greater reaction time is always money in the wallet!
- The higher rotor RPM also reduces the height loss in transitioning to safe OEI flight –something significant while operating to offshore decks where this ‘drop down height’ versus height available is crucial for a safe flyaway.
- Higher rotor RPM translates to higher tail rotor energy through its gear ratio thereby enhancing tail rotor authority – a fringe benefit for the bad day!
What if you don’t?
So what can you expect if you set theory aside and decide to undertake a take-off without selecting Nr to 102/103/104% as applicable to your helicopter? One of the helicopters I have flown carries this caution in the RFM:
“With NR increase function set to OFF, at given gross weight, a 10 ft penalty is to be applied on height loss determined with fly-away performance chart with NR 104%. At a given height loss, a 200 kg penalty is to be applied on gross weight determined with fly-away performance chart with NR 104%”
Of course, this is applicable to a particular type and cannot be assumed across the board. However, during offshore take-offs and landings be aware that 10 feet could well mean the difference between a safe getaway and an arrival on water. Then if the water doesn’t get you, the lawyers surely will!
Some aviators bemoan the cumulative stresses on airframe and engines (and consequent reduction in component lives) by use of 102% switch or the Cat A take-off procedure. However, I have not come across any data to support this apprehension and will be happy to review if somebody can table it. Having gone through the theory of engine failure and having been part of few engine failure flight tests, I am happy to use any functionality which helps me with rotor RPM, height and reaction time – all crucial factors when an engine fails!
Read, Discuss, Understand
Do visit the Rotorcraft Flight Manual relevant to the type you fly for details on engine failure and the elaborate graphs and procedures given therein for different take-off and landing profiles to get a thorough understanding of this condition. When the time comes, you don’t want to be up against a rotor that slows and time that flies!
©KP Sanjeev Kumar, 2017. All rights reserved.
Note: These are generic descriptions. Please refer to and abide by the Rotorcraft Flight Manuals applicable to the type you fly. Views expressed are personal and open to debate with a view to achieve better understanding of engine failure dynamics.