There are veterans of Britten-Norman Trislander operations who would argue that the best thing that could happen to the damn thing is that one of the engines would shut down, at least then you could hear yourself think. Although you’d then have plenty to think about. Ideally it would be the weird one up on the fin, just for stability.
However, you’d want it to happen in a slightly more controlled fashion than in this incident yesterday which involved the propeller making an unexpected attempted entrance to the cabin. Extraordinary in-flight slide-show here, and full story here.
I suppose it’s the ideal Trislander engine-loss really – the noise goes away and so does the prop-drag.
Some of you will be familiar with the splendid spoof Islander flight assessment that emanated from the Cairns neck of the woods some years ago. Follows below. If you haven’t read it, it’s a don’t-miss.
Undaunted by technical realities, the design team at Pilatus Britten -Norman has announced plans for the BN2-XL, promising more noise,reduced payload, a lower cruise speed, and increased pilot workload.
We spoke to Mr. Fred Gribble, former British Rail boilermaker, and nowChief Project Engineer. Fred was responsible for developing manyoriginal and creative design flaws in the service of his formeremployer, and will be incorporating these in the new BN2-XL technologyunder a licensing agreement. Fred reassured BN-2 pilots, however, thatall fundamental design flaws of the original model had been retained.Further good news is that the XL version is available as a retrofit.
Among the new measures is that of locking the ailerons in the centralposition, following airborne and simulator tests which showed thatwhilst pilots of average strength were able to achieve up to 30 degreesof control wheel deflection, this produced no appreciable variation inthe net flight of the aircraft. Thus the removal of costly andunnecessary linkages has been possible, and the rudder has beennominated as the primary directional control. In keeping with this newphilosophy, but to retain commonality for crews transitioning to theXL, additional resistance to foot pressure has been built in to therudder pedals to prevent over-controlling in gusty conditions (definedas those in which wind velocity exceeds 3 knots). An outstandingfeature of Islander technology has always been the adaptation of theO-540 engine, which mounted in any other aircraft in the free world(except the Trislander)is known for its low vibration levels. The Islander adaptations causeit to shake and batter the airframe, gradually crystallise the mainspar, desynchronise the accompanying engine, and simulate the sound offifty skeletons fornicating in an aluminium dustbin.
PBN will not disclose the technology they applied in preserving thiseffect in the XL but Mr. Gribble assures us it will be perpetrated inlater models and sees it as a strong selling point. “After all, theConcorde makes a lot of noise” he said, “and look how fast that goes.”However design documents clandestinely recovered from the PBN shredderhave solved a question that has puzzled aerodynamicists and pilots formany years, disclosing that it is actually noise which causes the BN2to fly. The vibration set up by the engines, and amplified by theairframe, in turn causes the air molecules above the wing to oscillateat atomic frequency, reducing their density and creating lift. This canbe demonstrated by sudden closure of the throttles, which causes theaircraft to fall from the sky. As a result, lift is proportional tonoise, rather than speed, explaining amongst other things theaircraft’s remarkable takeoff performance. In the driver’s cab (asGribble describes it) ergonomic measures will ensure that long-term PBNpilots’ deafnessdoes not cause in-flight dozing. Orthopaedic surgeons have designed acockpit layout and seat to maximise backache, en-route insomnia,chronic irritability and terminal (post-flight) lethargy.
Redesigned “bullworker” elastic aileron cables, now disconnected fromthe control surfaces, increase pilot workload and fitness. Specialnoise retention cabin lining is an innovation on the XL, and it ishoped in later models to develop cabin noise to a level which willenable pilots to relate ear-pain directly to engine power, eliminatingthe need for engine instruments altogether.
We were offered an opportunity to fly the XL at Britten-Norman’sdevelopment facility, adjacent to the BritRail tearooms at LittleChortling. (The flight was originally to have been conducted at thePilatus plant but aircraft of BN design are now prohibited fromoperating in Swiss airspace during avalanche season). For our missionprofile, the XL was loaded with coal for a standard 100 nm trip withBritRail reserves, carrying one pilot and nine passengers to maximisediscomfort. Passenger loading is unchanged, the normal under-wingprotrusions inflicting serious lacerations on 71% of boardingpassengers, and there was the usual confusion in selecting a doorappropriate to the allocated seat. The facility for the clothing ofembarking passengers to remove oil slicks from engine cowls duringloading has been thoughtfully retained.
Start-up is standard, and taxiing, as in the BN2 is accomplished bybrute force. Takeoff calculations called for a 250-decibel powersetting, and the rotation force for the (neutral) C of G was calculatedat 180 ft/lbs of backpressure. Initial warning of an engine failureduring takeoff is provided by a reduction in vibration of the flightinstrument panel. Complete seizure of one engine is indicated by themomentary illusion that the engines have suddenly and inexplicablybecome synchronised. Otherwise, identification of the failed engine isachieved by comparing the vibration levels of the windows on eitherside of the cabin. (Relative passenger pallor has been found to be anunreliable guide on many BN2 routes because of ethnic consideration).
Shortly after takeoff the XL’s chief test pilot, Capt. Mike “Muscles”Mulligan demonstrated the extent to whch modern aeronautical design hasleft the BN2 untouched; he simulated pilot incapacitation by slumpingforward onto the control column, simultaneously applying full rightrudder and bleeding from the ears. The XL, like its predecessor,demonstrated total control rigidity and continued undisturbed. Powerwas then reduced to 249 decibels for cruise, and we carried out somecomparisons of actual flight performance with graph predictions. At5000 ft and ISA, we achieved a vibration amplitude of 500 CPS and 240decibels, for a fuel flow of 210 lb/hr, making the BN2-XL the mostefficient converter of fuel to noise after the Titan rocket. Exploringthe Constant noise/Variable noise concepts, we found that in a VNEdive, vibration reached its design maximum at 1000 CPS, at which pointthe limiting factor is the emulsification of human tissue. Thecatatonic condition of long-term BN2 pilots is attributed to thissyndrome, which commences in the cerebral cortex and spreads outwards.We asked Capt. Mulligan what he considered the outstanding features ofthe XL. He cupped his hand behind his ear and shouted “WHAT?” Wereturned to Britten-Norman convinced that the XL model retains themarque’s most memorable features, whilst showing some significant andworthwhile regressions. PBN are not, however, resting on their laurels.Plans are already advanced for the TrislanderXL and noise tunnel testing has commenced. The basis of preliminarydesign and performance specifications is that lift increases as thesquare of the noise, and as the principle of acoustic lift is furtherdeveloped, a later five-engined vertical take-off model is also apossibility.”