Decreasing your exposure to risk
1. Don't fly real fast
Rev. 6 — page content was last changed 18 December 2009
consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au
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Decreasing your exposure to risk |
1. Don't fly real fastRev. 6 — page content was last changed 18 December 2009 consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au |
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Machines that move very fast seem to hold some kind of fascination for us. We like the thrill of driving fast cars and flying fast aircraft. But speed has its dangers. What are the dangers of flying too fast?
Here is an extract of an email from Italy to an RA-Aus member discussing the crash of a Flight Design CT recreational light aircraft: "The airfield has been closed due to a fatal accident with a CT that went over Vne and (broke up) just 50 metres above the airfield. I don't know if you ever experienced the CT, never fly it with pilots that want to show you how fast it is. I saw ... what happens if you feel greater than aircraft limits." The theme common to the difficulties encountered in that and similar disasters, is when moving really fast there will be little warning and no time to do anything. The results are invariably fatal! 1.1 How fast is too fast?The term 'very high speed' is entirely relative. In an aluminium tube and fabric aircraft it might be 70 knots; in an aircraft that cruises at 100 knots, excessive speed in favourable atmospheric conditions might be 140 knots.The airspeed that constitutes 'too fast' can change depending on the load carried and how it is distributed.There are limits to the payload an aircraft may carry safely and load must be distributed so that the aircraft's balance — the position of the aircraft's centre of gravity — is contained within defined limits. In addition there is a maximum safe operating weight permitted by the aircraft designer, or by regulation, for all flight conditions. So, before you load a 100 kg passenger and 30 kg of gear into an aircraft with full tanks, check the weight and balance charts in your Pilot's Operating Handbook or aircraft flight manual.Weight and balance affect control and stability at all speeds. Excess weight reduces the designed structural load limit factors. Cg positions outside the designated fore and aft limits may enhance elasticity reactions to aerodynamic loads, or reduce controllability because of moment arm changes, or delay (even prevent) recovery from unusual and/or high-speed situations. How are airspeed limits, especially Vne, determined?In FAR Part 23, Vd — the 'design diving speed' — is required to be not less than 1.4 times the design cruise speed for a normal category aircraft. To receive certification, it must be demonstrated, possibly by analytical methods, that at Vd, the propeller, engine, engine mount, and airframe will be free from over-speeding, severe vibration, buffeting, control reversal and most importantly flutter and divergence. To provide some safety margin, Vne (the IAS that should never be exceeded in level flight, descent or other manoeuvre) is set at 90% of the lower of Vd or Vdf. Vdf is a diving speed that has been demonstrated by a test pilot without problem in test flights, and which must be lower than or equal to Vd.Vne is always specified in the pilot's documentation as an indicated airspeed and is usually marked on the ASI (the red line). But unlike the performance airspeeds (also specified as indicated airspeeds and perhaps marked on the ASI), Vne is related to those structural characteristics and limitations associated with bending, twisting and flexibility, and which affect stability, control and even structural integrity. Limiting speeds are also associated with structural reaction to pilot-induced loads and to gust-induced loads. Limiting speeds could also be associated with other potential problems; for example, suction effects at particular speeds and attitudes might lead to canopy departure, or door or cowling security problems. Quite often, Vne is limited by the critical flutter speed; see 'Aerodynamic reactions to flight at excessive speed'. Does Vne stay the same no matter how high you fly?For most recreational light aircraft only one Vne is specified in the Pilot's Operating Handbook or aircraft flight manual. That value is probably conservative and applicable for operations below 10 000 feet amsl. The designers of most piston-engined GA aircraft specify one fixed-value Vne for operations up to the service ceiling; that value is represented by the fixed red line on the ASI. However, a minority of GA aircraft have supplementary lower-value Vne for operations in altitude bands above a stated altitude — perhaps above 10 000 or 15 000 feet. This approach to Vne specification is common with sailplanes.A very few aircraft designers select a true air speed value as a limiting airspeed applicable from sea level. FAR Part 23.1545 (c) requires that "If Vne varies with altitude, there must be means to indicate to the pilot the appropriate limitations throughout the operating altitude range". The 'means' is normally a placard next to the ASI. So, in such circumstances, designers must specify a series of Vne values, corresponding with all possible operating altitude bands. For example, the RA-Aus registered Pipistrel Sinus has the altitude capability of 28 500 feet and can build speed rapidly even in a shallow descent. For their own reasons (possibly associated with the flutter potential of the lightweight, high aspect ratio wings) the Pipistrel designers have deemed it wise to limit maximum speed to a particular TAS. The following table reflects the Sinus flight manual and the ASI placard; the maximum true airspeed target is 122 knots.
If there is insufficient manufacturer's information available for the aircraft you fly — and you are uncertain about the appropriate Vne for an operating altitude — then multiply the density altitude, in thousands of feet, by a factor of 1.5 to get the percentage decrease to apply to the specified Vne to establish a safe Vne appropriate to the altitude. For example if density altitude is 8000 feet and specified Vne is 100 knots then 8[000] × 1.5 = 12%. Corrected Vne = 88% of 100 = 88 knots IAS/CAS. 1.2 I like flying my aircraft fast. If I stay below Vne, I won't have to worry about structural failure, right?Vne is assessed at or near MTOW, with the cg within the fore and aft limits for the aircraft's specified category; it does not apply if weight, manoeuvring loads or cg position are outside the specified limits. As a maximum airspeed it applies only in smooth atmospheric conditions, for gentle control movements and symmetrical aerodynamic loads; even gusts associated with mild turbulence or control surface movements greater than perhaps a few degrees travel will lead to some nasty surprises, if operating close to but below Vne. At high speed the controls are very effective, with a probability of over-control applying extreme loads to the structures. Asymmetrical aerodynamic loads, such as combined rolling and pitching, reduce the maximum allowable airframe load by perhaps 30%. Take care because some aircraft control systems provide inadequate feedback of the load being exerted; i.e. a high load can be applied with a relatively low stick force.(The effect of gust loads is expanded in the section on wind shear and turbulence.) If an aircraft is operated within its specified flight envelope and weight and balance limits — observing the limiting accelerations and control movements, and maintaining airspeeds commensurate with atmospheric conditions — then the only possibilities of inflight structural failure relate to:
Be aware: deliberately exceeding Vne is the realm of the test pilot — who always wears a parachute! The following text is an extract from an RA-Aus accident investigation report: "(Witnesses) observed the aircraft in a steep dive at what appeared to be full power. The port wing appeared to detach from the aircraft …
1.3 How strong are the aeroplanes we fly?Aircraft structures, engineered by aeronautical professionals, are designed for adequate strength and stiffness while being as lightweight as possible. To receive type approval certification the design of an aircraft must conform with certain standards, including the in-flight manoeuvring loads plus the turbulence or gust-induced loads that the structure must be able to sustain for the category in which the aircraft may be operated. Even if not seeking certification the designer would still conform to the standards, but this may not apply to those aircraft not designed/engineered by professionals.FAR Part 23 is a recognised world standard for light aircraft certification and the following is an extract: "... limit loads [are] the maximum loads to be expected in service [i.e. the highest load expected in normal operations] and ultimate loads [are] limit loads multiplied by [a safety factor of 1.5]. [FAR Sec. 23.301] … The structure must be able to support limit loads without detrimental, permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation. … The structure must be able to support ultimate loads without failure for at least three seconds …" [FAR Sec. 23.305] In FAR Part 23 the minimum limit load factors that an aircraft must be designed to withstand at maximum take-off weight are: • +3.8g to −1.5g (or −1.9g) for the normal operational category (which would include most factory-built recreational aircraft); • +4.4g to −1.8g (or −2.2g) for the utility category (which includes most GA, and perhaps some RA, training aircraft); and • +6.0g to −3.0g for the acrobatic (i.e. aerobatic) category. There is an increasing risk of structural failure when exceeding the limit load factors, and each instance of excessive loading will accumulate airframe strain and add to the failure risk. We use load factors in terms of both g and total wing loading. Remember that aerodynamic forces increase with the square of the true airspeed; i.e. dynamic pressure = ½rV². There is an amplification of this relating to gust-induced loads, rather than manoeuvring loads, in the article 'Wind shear and turbulence'. Notes: 1. Uncertificated minimum recreational light aircraft, even with their low wing loading, can readily be overstressed just by flying straight and level at maximum speed and increasing load in a pull-up (positive g) or a full push-over (negative g). 2. Many GA aircraft are type certificated in both normal and utility category, and some are certificated in those plus the acrobatic category. In such cases the MTOW, cg limits and limit load factors are not fixed values but vary according to the intended flight operating category. Airframe elasticityAll aircraft structures exhibit some degree of elasticity. That is, they deflect a little, changing shape — flexing, bending and/or twisting — under applied aerodynamic loads. Those short-lasting structural distortions also contribute to a change in the aerodynamic forces, so the distortions and forces are mutually dependent. This is particularly so with the wings, tailplane and control surfaces. However, structures usually spring back to the normal position when the load is removed. Aeroelasticity may lead to some problems at high speed, but reducing elasticity means increasing rigidity, which perhaps involves an unwarranted increase in structural weight. So, aircraft structural engineering must be a compromise between rigidity and elasticity. See the notes on 'stress and strain' in the 'Builders guide to aircraft materials'.Aircraft flight envelopeThe V-n (or V-g) diagram below is a typical representation of a few aspects of a three-axis aircraft's flight envelope. It displays the aerodynamic load factor on the vertical axis — in terms of g acceleration units — between the certificated limit loads for a normal category aircraft of +3.8g to −1.5g, and airspeed along the horizontal axis emanating from the zero g position. The load is that which parallels the aircraft's 'normal' axis (hence V-n); i.e. the load at right angles to both the longitudinal and lateral axes in erect (i.e. not inverted) flight. The structural load limits shown are for symmetrical airframe loading only. They don't apply if a manoeuvring load is asymmetrical; for example, rolling or yawing, while pulling back on the control column, can place excessive loads on parts of the airframe. Asymmetrical loadings might reduce the acceptable limit load by 30%.
The 'clean' three-axis aircraft can be flown within the speed and load limits of the green region at any time although it is only possible to manoeuvre a light aircraft in the reduced-g band between +1g and –1g for seconds rather than minutes. Controlled flight at speeds less than the Vs1g stall speed may be accomplished with any manoeuvre that 'unloads' the wings; for example, 'push-overs', which reduce apparent weight (make you feel light in the seat) or banked turns where the aircraft is allowed to fall freely. Aerobatic aircraft can be pulled into a full-power vertical climb where the aoa is held close to the zero lift (zero load) aoa until the airspeed drops close to or below Vs1, then rudder and the slipstream energy is used to cartwheel the aircraft through a 180° hammerhead turn into a vertical descent. The aircraft will stall if flight is attempted outside those aerodynamic load/speed limits defined by the curved lines. It can be operated above the Va manoeuvring speed within the yellow region in reasonably smooth air conditions, as long as control deflections are applied slowly and smoothly and limited to perhaps 30% travel. If operating in the red region outside the structural load limits, or at velocities greater than Vd, structural distortion then failure may result. The more abrupt the application of airframe loads while operating in the red region, the greater the possibility of structural failure. No aircraft should be operated in the pink region above Vne because of aerodynamic reactions to excessive speed. The following are extracts from a report concerning certain engineering aspects of a fatal accident involving a Skyfox CA22. The aircraft had taken off from an airfield some 20 km from the accident site. The aircraft was seen to break-up in flight while overflying the pilot's house. The port aileron (or a portion of it) and the port wing were seen to detach from the aircraft and descend separately and relatively slowly. The fuselage with the starboard wing attached struck stony ground at high speed. Conclusions: "The most probable primary cause of failure was exceedence of the aircraft's structural design envelope, primarily in regard to speed in conjunction with negative load factor due to a gust, leading to compression failure of the forward strut. Aileron flutter, due to an out-of-balance condition, may have been a factor. It seems probable that the aircraft was flying close to, or above, its Vne of 93 knots. The permissable flight envelope is very small, and would not be at all difficult to exceed inadvertently, especially in a shallow descent." | ||||||||||||||||||||||
1.4 Perilous aerodynamic reactions to excessive speed: flutter and other booby trapsWing structures are akin to a 'tuning fork' extending from the fuselage. When a tuning fork is tapped the fork vibrates at a particular frequency; the stiffer the structure, the higher its 'natural' frequency. The natural frequency of a wing or tailplane structure may apply a limiting airspeed to flight operations that is related to structural instabilities: flutter and wing divergence.When airflow around a wing or control surface is disturbed by aerodynamic reactions or pilot inputs, the structure's elastic reactions may combine as an oscillation or vibration of the structure (possibly evident as a buzz in the airframe), which will quickly damp itself out at normal cruise speeds. (A test pilot might just 'tap' the control column when checking for onset of pre-flutter vibrations). At some higher speed — the critical flutter speed — where the oscillations are in phase with the natural frequency of the structure, the oscillations will not damp out but will resonate and rapidly increase in amplitude. (Pushing a child on a swing is an example of phase relationships and amplification). This condition is flutter and, unless airspeed is very quickly reduced by closing the throttle, the severe vibrations will cause wing, aileron or empennage control surface separation within a very few seconds. For more information on flutter see 'Aerodynamic reactions to flight at excessive speed'. Twisting the wings off!Wing divergence refers to a state where, at the very low angles of attack at high speed where the nose-down pitching moment is already very high, pressure centres develop pushing the front portion of the wing downward and the rear portion upward. This aerodynamic twisting action on the wing structure — while the rest of the aircraft is following a flight path — further decreases the aoa and compounds the problem; finally exceeding the capability of the wing/strut structure to resist the torsional stress and causing the wing to separate from the airframe — with no warning! This could be brought about if a gust is encountered at high speed.High-speed control reversal: will it always roll in the direction you want?As airspeed increases, control surfaces become increasingly more effective, reaching a limiting airspeed where the aerodynamic force generated by the ailerons, for instance, is sufficient to twist the wing itself. At best this results in control nullification; at worst it results in control reversal. For example if the pilot initiates a roll to the left the downgoing right aileron might twist the right wing, reducing its aoa, resulting in loss of lift and a roll to the right, probably with asymmetric structural loads: all of which would make life difficult when attempting to roll the wings level during the recovery from a high-speed dive. This could be exacerbated if the wing incorporates significant twist or washout, because the aoa of the outer section could be reduced below the normal zero -lift aoa and thus reverse the lift force on that section. Spars may fracture under those conditions.Many of the uncertified minimum ultralights, and perhaps some of the certificated aircraft, have low torsional wing rigidity that will not only make the ailerons increasingly ineffective with speed (and prone to flutter), but also will place very low limits on Vne and allowable wing loadings. Vne may be so low that it can be readily achieved in a shallow descent at normal cruise power. The problem is that in some home-built aircraft Vne may not be known and could be unexpectedly low! Wing washout: handy at low speed, not so good at high speed!Wings incorporating geometric washout have a significantly lower aoa towards the wing tips. At high speed when the wing is flying at low aoa there are high aerodynamic loads over the wings. But, the outer sections could well be flying at a negative aoa and the reversed load in that area — or just a badly distributed load due to the wing shape — will bend the outer wings down, possibly leading to outer spar fracture.Note: it can happen to certified GA aircraft; two recent (Victoria 2007 and Tasmania 2004) high-speed crashes of Shrike Commander aircraft both exhibited simultaneous negative load failure of both main wing spars at their outer splice joints. This aircraft incorporates 6.5° washout. The atmosphere will demonstrate how puny you are: vertical gust shear and gust loadsThe effective angle of attack of an aircraft encountering an atmospheric gust with a significant component parallel to the aircraft's normal axis (updrafts, thermals, down-currents, downdrafts, microbursts, macrobursts and lee waves) will be momentarily increased if the air movement is upward relative to the aircraft's flight path, or momentarily decreased if the air movement is downward. Thus an updraft will increase CL and lift causing an upwards acceleration of the aircraft, the magnitude of which is largely determined by the aoa change, the aircraft speed (higher speed — greater g load), the design wing loading and the aspect ratio. The higher the aspect ratio, the greater the change in load factor for a given increase in aoa and the easier it is to overstress the wings at high speed.The effects of shear and gust loads are expanded in the section on wind shear and turbulence. And there are other effects to think about!It is not just the preceding items that may provide problems at high speed. The maximum speed may be limited by the ability of the fuselage to withstand the bending moments caused by the loads on the tailplane necessary to counter the wing's high nose-down pitching moment at very low aoa.Also when nearing the zero-lift angle of attack in a high-speed descent, many cambered wings suddenly experience a very strong nose-down pitching moment and the aircraft will 'tuck under' or 'bunt' rapidly. This instability will certainly make any pilot wish they had not been flying so fast. The symmetrical aerofoil wings often used in aerobatic aircraft don't have this problem. Also the possibility of a runaway propeller in a fast dive is always there for those aircraft with a constant-speed propeller governor. There is nothing much you can do about that except close the throttle and reduce to minimum flight speed by easing the nose up. Once everything is settled down fly slowly, consistent with the default fine pitch stop blade setting, to a suitable airfield using minimum throttle movements. Another problem is the possibility for extreme loads to be applied in a high-speed pull-up. You can also induce structural damage at moderate speeds!Excessive speed is not always a factor in an aircraft structural failure. In Britain, June 2007, a 900-hour Europa Classic (a type that is represented in the RA-Aus aircraft register) suffered an in-flight break-up. Witnesses said the aircraft had been flying normally but then the tailplane started to make significant up-and-down movements. Then the horizontal stabilisers detached from the tail, and the wings folded up before separating from the fuselage. The engine stopped and the aircraft plummeted to the ground. The primary cause was probably tailplane flutter, possibly initiated by excessive play developing between the stabiliser torque tube and a mass balance arm.Also, for example, mishandled manoeuvring of weight-shift aircraft can lead to a very fast-acting and uncontrollable pitch autorotation or tumble that imposes extreme transient loads on the structure. The following is a condensed version of an Australian Transport Safety Bureau 'Technical Analysis Occurrence Report' into three fatal accidents. Note: the Coroner's findings in relation to the fatal accident near Atherton does not support any view that the accident was caused by pilot mishandling; rather, the Coroner's "preference is towards port side wing tip separation as a consequence of the un-airworthy state of the aircraft ..." An Airborne Edge trike impacted terrain near Atherton, Qld during a 2005 flight. In 2006 a similar Airborne Edge aircraft impacted terrain at Cessnock, NSW. In both instances, RA-Aus initiated safety investigations during which similarities in the structural failures of both aircraft were observed. In addition, a third accident occurring in 1996 near Hexham involving an HGFA registered Airborne aircraft with similar structural failure was identified.
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1.5 How should I recover from flight at excessive speed?Generally excessive speed can only build up in a dive, though just a shallow dive can build speed — and rate of descent — quite quickly. Table 2 is a calculation of the rate of descent after a few seconds at dive angles of 10°, 30° and 45° — for a moderately slippery light aircraft. It can be seen that even a non-turbocharged aircraft entering a 10° or 15° descent from 8000 feet or so could quickly be exceeding Vne.
Recovery from an inadvertent venture into the realm of flight near, or even beyond, Vne is quite straight-forward but requires pilot thought and restraint when initiating the recovery procedure, particularly so if the aircraft is turning whilst diving. Considerable height loss will occur during recovery, so restraint is required when the ground is rapidly expanding in the windscreen.
A problem with this procedure is that most light aircraft do not have an accelerometer fitted, so it is difficult to judge the g being pulled. However, if properly executed 60° steep turns are practised then you can gain some idea of the 2g load on your own physiology. At the higher end, the average fit person will probably start feeling the symptoms of greyout by 4g. Recovery from a spiral diveIn a well-developed steep spiral dive, the lift being generated by the wings (and thus the aerodynamic loading) to provide the centripetal force for the high-speed diving turn, is very high. The pilot must be very careful in the recovery from such a dive, or excessive structural loads will be imposed. If back elevator force is applied to pull the nose up while the aircraft is turning, the result will be a tightening of the turn and rapid increase in rate of descent — thus further increasing the aerodynamic loading or possibly prompting a very high speed stall.Reducing power and levelling the wings must start first, with the rudder and elevators held in the neutral position. As the wings become level with the aircraft still diving at high speed, all the lift that was providing the centripetal force may now be directed vertically (relative to the horizon) and if up elevator is applied the aircraft may start a rapid high g pitch-up — even into a half loop. Thus to prevent this the pilot must hold the elevators in the neutral position while rolling level and even be prepared to start applying forward stick pressure even before the wings become level. Remember: the theme common to all problems encountered when moving at very high speed is that there is little or no warning, and little time to do anything about it! The ONLY safe procedure is not to push the high - end of the envelope at any height, don't exceed Va manoeuvring speed if the atmosphere is exhibiting any other than very light turbulence, and keep the aircraft airworthy. The next article in this series discusses the stall/spin accident. |
'Decreasing your exposure to risk' modules
| Introduction | Don't fly real fast | Don't stall and spin in from a turn | Don't land too fast in an emergency |
| Engine failure after take-off | The turn back, possible or impossible — or just unwise? | Wind shear and turbulence |
Copyright © 2007–2009 John Brandon [contact information]
