Factors Affecting Safe take-off Performance

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Factors Affecting Safe take-off Performance

Post  Admin on Tue Dec 01, 2009 6:44 am

Apart from the pilot's condition and capability, take-off performance is limited by the following constraints, all of which should be carefully assessed within pre take-off procedure to establish whether a safe take-off is viable.

Aircraft weight and balance. The critical nature of aircraft weight and balance at take-off has been highlighted in the 'Weight and balance' module, and should be reviewed.

Standard take-off distance [TOD]. TOD should always be expressed as the total distance required to accelerate from a standing start, and clear an imaginary screen 50 feet (15.2 m) high. The ground roll is that first part of the TOD where the aircraft's weight is partly or fully supported by the undercarriage; sometimes people incorrectly refer to the ground roll as the TOD, ignoring the fact that the distance covered from the lift-off point to climb to 50 feet may be longer than the ground roll. It is known for an under powered aircraft to be able to lift-off but then be unable to climb out of ground effect.

TOD is officially expressed as the take-off distance required [TODR] to clear the 50 feet screen.These standards require that the operating conditions associated with a particular TODR will be specified in approved aircraft take-off performance charts. These conditions are pressure altitude, temperature, runway slope and surface, and wind velocity.

CAO 101.28, an airworthiness certification requirement for commercially supplied amateur-built kit ultralights, states in part (at paragraph 3.6):
"The take-off distance shall be established [by the manufacturer] and shall be the distance required to reach a screen height of 50 feet from a standing start, … appropriate to a short dry grass surface …
[The] aeroplane [should reach] the screen height at a take-off safety speed [author's emphasis] not less than 1.2 Vs1 … Take-off charts … shall schedule distances established in accordance with the provisions of this paragraph, factored by 1.15."

CAO 95.55 has much the same wording but specifies 1.3 Vs1 as the take-off safety speed and FAR Part 23 is similar.

'Short dry grass' means grass less than 100 mm long.

Unless the manufacturer's take-off performance figures are published as an approved performance chart within the aircraft's flight manual or comparable document, then such figures should be treated as unverified sales claims. In the absence of any specified conditions in an unapproved performance chart, assume that sea level ISA, nil wind and smooth, dry runway are the basis for the published data.

If the manufacturer's performance charts only provide data for the aircraft at maximum take-off weight then, for an ultralight aircraft, a reduction of 10% in TODR for each 50 kg the aircraft's weight below MTOW is probably a reasonable estimate.

Stopping distance required. The distance required to reach flight speed, and then bring the aircraft to a halt, should be known. It may be necessary to abandon the take-off soon after lift-off, due to doubtful engine performance or other event — this is particularly important in short field or 'hot and high' take-offs. If take-off and landing distance (over a 50 feet screen) charts are available then the total distance needed to take-off, abandon take-off at 50 feet, land and bring the aircraft to a halt is the sum of the charted take-off and landing distances required.

Airframe condition. An airframe in a battered or dirty condition, or which sports unnecessary or non-standard accoutrements, will increase drag and retard acceleration, lengthen TODR, and reduce climb performance.

Engine age, condition and operating temperatures. An engine that is incapable of producing its rated power will reduce acceleration, lengthen TODR and reduce climb performance. The engine manufacturer's instructions regarding warm-up procedures should be followed, to ensure appropriate temperatures and pressures are established before the engine is subject to the stresses of take-off power; otherwise the potential for an 'engine failure after take-off' is greatly increased.

Propeller condition and pitch. Chipped leading edges or scored blades, apart from being dangerous due to the possibility of delamination or fracture, will adversely affect thrust output. Blade pitch at a coarse setting — a cruise setting — will reduce acceleration and climb performance.

Tyre pressure. Under-inflated tyres increase the rolling friction, decrease the acceleration and add perhaps 10% to the ground roll.

Airfield dimensions and slope. The usable length of runways or strips must be known, as well as the degree of slope. Taking off upslope will reduce acceleration and lengthen the ground roll because thrust must also overcome a force equal to the aircraft weight × the sine of the angle of slope, in addition to the drag and rolling friction. The ground roll will increase by about 15% for each 2% of upslope. Runway slope can be measured by taking an altimeter reading at each end, dividing the elevation difference by the runway length (in feet) and multiplying by 100 to get the approximate slope percentage.

Airfield surface and surrounds. A short dry grass or rough gravel surface might add 10% to the ground roll compared to that for a smooth, sealed surface. Wet or long grass might add 50% to the ground roll. A soft or waterlogged surface might double the ground roll. Surface water and/or wet grass can lead to aquaplaning and loss of directional control; the effect of frost is similar. The height of obstructions and local terrain must be known.

Airfield density altitude. The density altitude is a critical factor that is often not correctly assessed, and has a major effect on engine output, propeller performance and lift generated. Thus it affects acceleration, TODR and climb performance to such an extent that on 'hot and high' airstrips an aircraft may be incapable of safe take-off and climb-out. Re-read section 3.4 'High density altitude'.

Wind velocity and turbulence. After weight and balance plus density altitude, the major considerations in take-off performance for a properly maintained aircraft are then wind strength, direction, gradient, downflow, gust intensity, surface turbulence and the potential for wind shear events. Please read 'Surface gusts or low level wind shear' in the 'Wind shear and turbulence' module.

The diagram to the left indicates possible cumulative effects of some take-off conditions on TODR. But as explained in section 11.6, the take-off distance required can be much greater.

The pilot in command of an aircraft must assess all the foregoing factors and conditions to ascertain the cumulative total distance required for take-off and obstacle clearance, and then judge if the take-off can be conducted safely. The golden rule is "If you have ANY doubts, don't fly".

The most favourable conditions for optimum take-off performance at MTOW are:
a pilot who follows the rules and the recommended procedures
an aircraft in very good condition and fitted with a 'climb' or variable pitch propeller
a surface that is dry, smooth and level — or with a slight downslope
a low density altitude; i.e. low elevation and low temperature
a smooth, full headwind of reasonable and constant velocity
sufficient separation is maintained to avoid aircraft wake turbulence.

All information accredited to http://www.auf.asn.au/index.html on page http://www.auf.asn.au/groundschool/umodule11.html

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