Autorotation, EOL, and Gunsight Tracking

MS&T’s Europe Editor, Dim Jones, visited the Empire Test Pilots’ School (ETPS) at MoD Boscombe Down to track the progress of the members of the Class of 2019; by the time of this instalment, they were approaching the halfway mark, undertaking more complex and demanding tasks.

In 2003, QinetiQ and the UK MoD entered into a 25-year Long-Term Partnering Agreement (LTPA) to oversee various activities, one of which was ETPS. As time progressed, it became apparent to both the MoD and QinetiQ that ETPS could be run more cost-effectively without sacrificing the quality of the product, and a new ‘sub-LTPA’ was established in 2016 to manage the transformation of the syllabus and the fleet and to run the new course thereafter. The transformation was to be accomplished in two years, while still continuing to run the old course; this was a tall order, but the first of the new courses, the Class of 2019, commenced training on time and, on 26June this year, the new ETPS was formally opened by the outgoing Chief of the Air Staff, Air Chief Marshal Sir Stephen Hillier who, in his previous appointment as Deputy Chief of Defence Staff (Capability) had, alongside QinetiQ’s CEO, Steve Wadey, initiated the change programme.

The main ETPS course comprises fixed- and rotary-wing(FW and RW) elements, the former sub-divided into Fast-Jet (FJ) andMulti-Engine (ME) and, within each discipline, pilots and Flight Test Engineers(FTE). During my first visit, I concentrated on the FW syllabus, and flew inthe Avro RJ-70 on a Pressure Error Correction Measurement exercise. This time,the emphasis was on RW – emphatically not my area of expertise, although I havehad the privilege during my flying career of providing light entertainment forprofessional and properly qualified helicopter aircrew in a variety of rotarytypes.

The ETPS RW pilot syllabus is broadly the same as forFW, but with some subtle differences. UK Army, RN, RAF and internationalstudents must have a minimum of 750 hours flying experience and, these days,this will normally have been on a single operational type. The initial part ofthe course is a limited conversion to the ETPS types, such that the studentscan carry out the syllabus exercises. First, they investigate the performancecharacteristics in level flight (including low-speed and hover), and then inautorotation.

Next comes handling. Most modern helicopters have anAutomatic Flight Control System (AFCS) which gives artificial feel; the initialinvestigation of basic stability and control is with the AFCS disengaged, andcovers longitudinal and lateral stability in forward flight, and at low speedand in the hover. These exercises are then repeated with the AFCS engaged,before progressing to the study of High Order Flight Control Systems (HOFCS),using a specially modified variable-stability aircraft (normally a Bell 205 or412 leased from a Canadian operator) in which, within a calculated safetyenvelope, the flight control ‘gains’ (the flight control response to a givenstick input) and AFCS configuration can be varied by inputs from the crew, andevaluated.

The syllabus then progresses to autorotation, enginefailure, engine-off landing (EOL) and the ‘Avoid Curve’. In autorotation, a helicopter is driven only by aerodynamic forces – no power fromthe engine. The engine is disengaged and the rotor blades are driven solely bythe upward flow of air through the rotor. It isthe means by which a helicopter can land safely in the event of complete enginefailure. During autorotation,maintenance of the main rotor rpm (NR) is critical, since it isthrough converting the energy thus stored, added to that gained through forwardspeed, that the rate of descent can be reduced when approaching the ground, andthe aircraft flared to achieve a safe landing. All but the simplest helicoptershave some sort of an automatic power control system, whereby the power will beautomatically adjusted to maintain NR with varying collectiveinputs. Autorotation performance is assessed at varying forward speeds, andalso at reduced NR, which will reduce the rate of descent, butcorrespondingly the rotor energy available to control the final stages of thelanding.

The next stage is EOL (Engine Off Landings); somestudents will not have much experience of this, since increasingly – as at theUK’s Defence Helicopter Flying School (DHFS) – training aircraft are nowtwin-engined, and some familiarization with EOL techniques will be requiredbefore progressing to the syllabus sorties. Stage 1 is the study of enginefailure characteristics, and particularly the effects of varying the reactiontime to a failure – also known as lever delay – on aircraft handling andperformance. This leads to the formulation of the best EOL profile for theaircraft being tested and, from this, identification of the Avoid Curve – theparameters outside which, for whatever reasons, a safe EOL would becompromised.

AnInternational Sortie

The syllabus sortie which I followed through was thefirst covering autorotations, and would be flown in the single-engined AirbusH125B3e. The crew would be a Flight Test Instructor, Ian, flying in theleft-hand seat (LHS), with his FT Student (Michael, from the Swiss ArmedForces) in the RHS, and an FTE student (Dave, a Royal Australian Navy officer)in the back. The sortie would be monitored in Boscombe Down’s ground telemetrystation by Andy and Simon, by me and by Edgar, a RW FTE student from theRepublic of Singapore. The aims of Autorotation Sortie 1 were for the studentsto receive instruction in test methods commonly used to evaluate helicopterautorotative performance and handling characteristics, and to receive familiarisationtraining on the use of telemetry.

Ground telemetry screens at Boscombe Down, monitoring an autorotation scenario. Image credit: Dim Jones.

The H125 has two positions of the throttle twist gripon the collective lever: FLT will allow the engine control to vary the torque,thereby maintaining NR while responding to pilot performancedemands; while IDLE maintains the engine at flight idle, and the collective canbe used to control NR.

First, however, I was fortunate enough to get airbornein the LHS of the H125, and experience the aircraft’s handling and flight testenvironment. The FADEC-controlled Arriel engine gives it excellent performance,the all-round visibility is first class, and the aircraft is a delight to fly,with or without the stability augmentation system – although I am told thatfailure of the single hydraulic system renders the flight controls extremelyheavy. The glass cockpit includes a large Flight Test Instrumentation (FTI)screen on the left-hand panel, on which can be brought up a wide variety of FTIdisplays; there is provision for an iPad Mini mounted on the RHS coaming, andtablets can also be mounted in front of the rear seats.

Technology does not always make life simpler, however:setting the height warning bug on the radar altimeter, which used to involvethe rotation of a single knob, now requires a lengthy sequence ofbutton-presses. My personal minder, Eric, showed me a cross-section of the125’s capabilities, including autorotation and simulated EOL, and, under hiswatchful eye, I duly reprised my airborne entertainment function; alas, asalways when the fun-meter is pegged in the right-hand corner, the time passedall too quickly.

The Autorotation 1 sortie comprises six serials, allstarting from around 8000 ft, with a minimum height of 1000 ft above ground,save for the last serial, in which the minimum height is 500 ft agl. The firstthree investigate various performance parameters at the standard autorotationspeed of 65 Knots Indicated Air Speed (KIAS), and target NR of 400;the acceptable range is 320-430. Serial 3 is then repeated with targetairspeeds of 90 and 50 KIAS. The next two serials look at a target NRof 360 at 65 and then 90 KIAS. For each serial, the FTE, who controls themission aspects of the sortie, records various engine and airframe parameters,such as collective position, actual NR, yaw pedal position, and rateof descent. In addition, the aircrew record any vibration encountered, using anobjective numerical assessment of its severity and frequency, ranging from‘None’ to ‘Intolerable’.

On the ground, the telemetry team monitor the sortie;their displays can be set to show the same as the airborne FTE is using, or anyother set of data available from the aircraft FTI equipment. On some syllabusexercises, notably assessment of the effects of varying reaction time to anengine failure (lever delay), telemetry monitoring can be safety-critical. Thefinal serial looks at turn performance in autorotation and, for a variety ofreasons, is flown at the best-rate-of-climb speed (VY). Despiteproblems on startup in both aircraft and ground telemetry (Traffic CollisionAvoidance System - TCAS) failure in the aircraft, comms and system issues intelemetry), the sortie progressed satisfactorily and achieved all its aims.

Formationsand Tracking

Calspan's Learjet 'in-flight simulator' enables evaluations in a real flight environment that includes fully representative motion cues and real-world visual cues. Image credit: Calspan.

Meanwhile, the FW course had progressed to their HOFCSmodule. They had already completed sorties which explored basic flight controllaws and characteristics in ‘non-augmented’ flight control systems; now theywere looking at pure fly-by-wire systems, how adjustments to the controlparameters can be used to modify the way the aircraft will respond to controlinputs, and how to test the modified system to ensure it produces the desiredresults, without displaying any adverse characteristics. To this end, the HOFCSsorties are flown in the Calspan Learjet, which is essentially a flyingsimulator. The LHS controls are standard Learjet, the control column is a yoke,and the aircraft – when flown from the LHS – behaves as a normal Learjet. TheRHS is equipped with two control stick-tops, one conventional centralcolumn-mounted and the other a sidestick. Both of these, under normalcircumstances, work to a standard flight control model which produces handlingcharacteristics similar to a Hawk. However, by use of switchery on the centreconsole, the crew can input variations to this standard model.

For all Calspan Learjet sorties, the LHS is occupiedby a fully qualified Learjet captain, who acts as the safety pilot. The RHS isoccupied by either a student Test Pilot or a student FTE. There are threesorties in the HOFCS module: one is a two-hour general handling sortie,dedicated to FTE students, and two them will share the stick time. The secondis also a general handling sortie, this time dedicated to a student Test Pilot,in which he has the opportunity to experience the effects of the flight controlsystem variations, supervised by a Flight Test Instructor in the jump-seatbehind the flight deck. During the final sortie, these effects are demonstratedin the context of two exercises: close formation, simulating the ‘waiting’position astern an air refuelling basket; and a simple guns tracking exercise,initiated from a perch position. For this sortie, the Learjet is joined by a‘tow’ aircraft, in this case an ex-Swiss Air Force Hunter Mk58, operated byHawker Hunter Aviation from RAF Scampton – still an iconic aircraft, not leastto the many, like me, who only managed enough Hunter hours to whet theappetite, but not to satisfy it!

A Swiss-model MK58 Hunter operated by Hawker Hunter Aviation, showing the 'Sabrinas' and gunports. Image credit: Erik Bruijns.

The variables to be investigated on the sortie are:Time Delay (between control input and effect); Phase Lead; Phase Lag; andAileron Rate Limiter. To the uninitiated, Time Delay and Phase Lag might appearto be much the same; however, I am assured that there are subtle differences. TheHunter, of course, has no AAR basket, so the visual references used to judgethe formation positions are the four Aden Cannon gunports under the Hunter’snose, and the rounded shell-case pods under the gun-pack – nicknamed‘Sabrinas’, in honour of the eponymous actress of the 50s and 60s. The Learjethas no HUD or gunsight, so ETPS have designed an ingenious modification, usingthe camera function of a mobile phone fitted above the RHS forward coaming, inwhich a simple gunsight picture is superimposed on the camera image. Pitchcontrol characteristics are evaluated during the approach to the ‘basket’ fromastern, or while manoeuvring to track the target from the perch; roll controlby laterally offsetting the aircraft to a point astern one of the Hunter’sunderwing tanks and then returning to the centerline, or by placing the Hunterat the three o’clock or nine o’clock position in the ‘gunsight’ and thenreadjusting to the tracking picture.

The exercise involves testing each of the first threevariables above for pitch and roll in each manoeuvre. The settings can beadjusted for the best results in one environment, and then carried over to theother; unsurprisingly, what’s good for guns tracking isn’t necessarily good forclose formation. The plot is further complicated by the fact that no two pilotsare the same, in terms of what they assess as being the optimum flight controlgains; generally speaking – but not exclusively – FJ pilots tend to be‘high-gain’ and ME pilots ‘low-gain’.

Whereas, under the right circumstances, Phase Lead andPhase Lag can enhance handling qualities, Time Lag is always bad, but anelement of it may be an inescapable result of the design of an FCS. The aim isto minimise it, and then assess whether the result is acceptable orunacceptable. Phase Lead or Phase Lag, if inappropriately applied, can resultin a Pilot-Induced Oscillation (PIO) which, in extreme cases, can lead to lossof control. Similarly, a rate limiter – which limits the rate at which acontrol response is applied, irrespective of stick input – can also induce aPIO. The results of these exercises are translated into objective assessmentsof Handling Qualities Ratings (HQR) and PIO susceptibility, using matrices andnumerical ratings developed for that purpose. The student Test Pilot for thesortie I monitored was, once again, our Australian F/A18 pilot, Aaron. Hereported that the sortie had gone well, that he had used the centre-stickrather than the side-stick throughout, and that he judged theaileron-rate-limiter function to be the most disconcerting effect, and the mostlikely to lead to a PIO.

AcceleratedDevelopment

When I visited, the Class of 2019 were about halfwaythrough the course and, despite this being the first of the new syllabus andoperating many new aircraft types, were less than a week behind the plannedschedule. To put this in context, QinetiQ’s Director Operations Air &Space, Simon Tate, averred that “you couldn’t say that of any other [ETPS]course in living memory”. Unsurprisingly, given the accelerated nature of thetransition to the new syllabus, minor ‘bumps in the road’, such as some early‘programming friction’ had been encountered. However, the course was on track,aircraft availability was excellent, the syllabus and exercises were underconstant review and development, and student feedback had been extremelypositive.

When I return for Part 3 (next issue of MS&T), the course will have completed their final Capstone projects and be gearing up for graduation.

PREREQUISITE: ETPS Part 1

My first report on ETPS, “Learn to Test, Test to Learn” in Issue 2019-4/5 of MS&T ( www.militarysimulation.training/articles/learn-to-test-test-to-learn/) provided a brief history and some background to the recent transformation of both the unit and the legacy course, specifically a major revision of the syllabus and teaching methods to comply with EASA qualification regulations, and the transfer of a significantly updated aircraft fleet from the military to the civil register.

ETPS formed in 1943 as part of the Aircraft and Armament Experimental Establishment (A&AEE), a military unit answering to the Air Ministry (later the Ministry of Defence (MoD) and collaborating closely with the Royal Aircraft Establishment (RAE) which became, in 1988, the Royal Aerospace Establishment. During the early 1990s, RAE and ETPS became part of the Defence Research Agency (DRA), later renamed the Defence Evaluation and Research Agency (DERA). In 2001 DERA was part-privatised by the MoD, resulting in two separate organisations, the state-owned Defence Science and Technology Laboratory (Dstl), and the privatised company QinetiQ.