FLIGHT SAFETY :: Aircraft Performance

The data needed to predict the performance in the expected conditions may be in any one of the following:

• The UK Flight Manual, or for a few older aeroplanes, the Performance Schedule.

The Pilot's Operating Handbook or the Owner's Manual. This is applicable to most light aeroplanes and sometimes contains CAA Change Sheets and/or Supplements giving additional performance data which may either supplement or override data in the main document, eg a ’fleet downgrade‘.

For some imported aeroplanes, an English language Flight Manual approved by the airworthiness authority in the country of origin, with a UK supplement containing the performance data approved by the CAA.

Many light aeroplanes are in performance group E, and certificated with UNFACTORED data, being the performance achieved by the manufacturer using a new aeroplane and engine(s) in ideal conditions flown by a highly experienced pilot. The CAA does not verify the Performance Data on all foreign aeroplanes; in some cases a single spot check is made.

To ensure a high level of safety on UK Public Transport flights, there is a legal requirement to apply specified safety factors to unfactored data (the result is called Net Performance Data). It is strongly recommended that those same factors be used for private flights in order to take account of:

- Your lack of practice
- Incorrect speeds/techniques
- Aeroplane and engine wear and tear

Less than favourable conditions

Performance data in manuals for UK manufactured aeroplanes certificated for the purposes of Public Transport may include the Public Transport factors, (i.e. Net Performance) but manuals and handbooks for the smaller aeroplanes often do not. For foreign manufactured aeroplanes the Net Performance may be included as a Supplement.

Any 'Limitations' given in the Certificate of Airworthiness, the Flight Manual, the Performance Schedule or the Owner's Manual/Pilot's Operating Handbook are mandatory on all flights. (Note that there can be a UK Limitation contained in a Supplement which is not referred to in the text of the main document.)

If any advice/information given in this leaflet differs from that given in the Flight Manual, (or Pilot's Operating Handbook), then you must always comply with the manual or handbook – these are the authoritative documents.

A list of variables affecting performance together with Factors for non-Public Transport operations are shown in tabular form at the bottom of this page. These represent the increase in take-off distance to a height of 50 feet or the increase in landing distance from 50 feet. It is intended that the tabular form will be suitable for attachment to a pilot's clipboard for easy reference.

When specific Factors are given in the aeroplane's manual, handbook or supplement, they must be considered the minimum acceptable. The primary source is the Flight Manual or Pilot's Operating Handbook but cross check using this where other Information is not available.

Aeroplane weight: use the actual aircraft Basic Empty weight stated on the Weight and Balance Schedule for the individual aeroplane you plan to fly. The weight of aeroplanes of a given type can vary considerably dependent upon the level of equipment, by as much as 77 kg (170 lb) – the invisible passenger, for a well equipped single engined aeroplane. Do not use the ‘example weight’ shown in the weight and balance section, it may be a new aeroplane with minimum equipment. Remember, on many aeroplanes it may not be possible to fill all the fuel tanks, all the seats and the baggage area.

Aerodrome elevation: performance deteriorates with altitude and you should use the pressure altitude at the aerodrome for calculations. (This equates to the height shown on the altimeter on the ground at the aerodrome with the sub-scale set at 1013 mb.)

Slope: an uphill slope increases the take-off ground run, and a downhill slope increases the landing distance. Any benefit arising from an upslope on landing or a downslope on take-off should be regarded as a 'bonus'. There are a ‘few one way strips’ where the slope is so great that in most wind conditions it is best to land up the hill and take off downhill.

Temperature: performance decreases on a hot day. On really hot days many pilots have been surprised by the loss of power in ambient temperatures of 30°C and above. Remember, temperature may be low on a summer morning but very high in the afternoon.

Wind: even a slight tailwind increases the take-off and landing distances very significantly. Note that if there is a 90° crosswind there is no beneficial headwind component and aircraft controllability may be the problem. Where the data allows adjustment for wind, it is recommended that not more than 50% of the headwind component and not less than 150% of the tailwind component of the reported wind be assumed. In some manuals these factors are already included, check the relevant section.

Cloudbase and visibility: if you have to make a forced landing or fly a low-level circuit and re-land, you MUST be able to see obstacles and the ground. Thus, cloudbase and visibility have to be appropriate.

Turbulence and windshear: will adversely affect the performance, you must be aware of these when working out the distances needed.

Surrounding terrain: if there are hills or mountains nearby, check that you will have a rate or angle of climb sufficient to out-climb the terrain. This is particularly important if there is any wind, it may cause significant down drafts.

Rain drops, mud, insects and ice: these have a significant effect on aeroplanes. Stall speeds are increased and greater distances are required. Note that any ice, snow or frost affects all aerofoils, including the propeller and also increases the aircraft's weight – you must clear it all before flight.

Tyre pressure: low tyre pressure (perhaps hidden by grass or wheel fairings) will increase the take-off run.

Engine failure: since an engine failure or power loss (even on some twin-engined aircraft) may result in a forced landing, this must be born in mind during all stages of the flight.

Cross wind: a cross wind on take off may require use of brakes to keep straight, and will increase the take off distance.

Decision point: you should work out the runway point at which you can stop the aeroplane in the event of engine or other malfunctions e.g. low engine rpm, loss of ASI, lack of acceleration or dragging brakes. Do NOT mentally programme yourself in a GO-mode to the exclusion of all else. If the ground is soft or the grass is long and the aeroplane is still on the ground and not accelerating, stick to your decision-point and abandon take off. If the grass is wet or damp, particularly if it is very short, you will need a lot more space to stop.

Engine failure: under limiting conditions an engine failure shortly after lift off may preclude continued flight and a forced landing will be necessary. Where the performance is marginal, the following points must be considered when deciding the best course of action:

- while flying with asymmetric power it is vital that airspeed is maintained comfortably above the minimum control speed, VMC. A forced landing under control is infinitely preferable to the loss of directional control with the aircraft rolling inverted at low altitude. If there are signs you are losing directional control, lower the nose immediately if height permits to regain speed and if all else fails reduce power on the operating engine. (Care must be taken to maintain normal margins above the stall.)

Performance and stall speed margins will be reduced in turns. All manoeuvres must be kept to gentle turns.

Use of available length: make use of the full length of the runway, there is no point in turning a good length runway into a short one by doing an ‘intersection’ take off. On short fields use any ‘starter strip’.

Rolling take off: although turning onto the runway, and applying full power without stopping can reduce the take off run, it should only be used with great care (due to landing gear side loads and directional control) and your propwash must not hazard other aircraft. If you are having to do this sort of thing, then the runway is probably TOO SHORT.

Surface and slope: grass, soft ground or snow increase rolling resistance and therefore the take-off ground run. When the ground is soft, a heavy aircraft may ‘dig in’ and never reach take off speed. Keeping the weight off the nosewheel or getting the tail up on a tail wheel aircraft, may help. For surface and slope, remember that the increases shown are the take-off and landing distances to or from a height of 50 feet. The correction to the ground run will usually be proportionally greater.

Flap setting: use the settings recommended in Pilots Handbook/Flight Manual but check for any Supplement attached to your manual/ handbook. The take-off performance shown in the main part of the manual may give some flap settings which are not approved for Public Transport operations by aeroplanes on the UK Civil Aircraft register. Do not use settings which are ‘folk-lore’.

Humidity: high humidity can have an adverse effect on engine performance and this is usually taken into account during certification; however there may be a correction factor applicable to your aeroplane. Check in the manual/ handbook.

Engine power: check early in the take off run that engine(s) rpm is correct. If they are low, abandon take off when there is plenty of room to stop. Brief use of carb heat at the hold will ensure carb ice is not forming.

When landing at places where the length is not generous, make sure that you touch down on or very close to your aiming point (beware of displaced thresholds). If you’ve misjudged it, make an early decision to go around if you have any doubts – don’t float half way along the runway before deciding.

Landing on wet grass, or snow, can result in increased ground roll, despite increased rolling resistance. This is because of the amount of braking possible is reduced, due to lack of tyre friction. Very short wet grass with a firm subsoil will be slippery and can give a 60% distance increase (1.6 factor).

When landing on grass the pilot cannot see or always know whether the grass is wet or covered in dew.

The landing distances quoted in the Pilot’s Operating Handbook/Flight Manual assume the correct approach speed and technique is flown, use of higher speed will add significantly to the distance required whilst a lower speed will erode stall margins.

Take-off It is strongly recommended that the appropriate Public Transport factor should be applied for all flights. For take-off this factor is x 1.33 and applies to all single engined aeroplanes and to multi-engined aeroplanes with limited performance scheduling (Group E). Manuals for aeroplanes in other Performance Groups may give factored data.

Pilots of these latter Performance Group aeroplanes and other complex types are expected to refer to the Flight Manual for specific information on all aspects of performance planning. It is therefore important to check which Performance Group your aeroplane is in.

The table at the end of this page gives guidance for pilots of aeroplanes for which there is only UNFACTORED data. It is taken from AIC 12/96 (Pink 120).

Don‘t forget, where several factors are relevant, they must be multiplied. The resulting Take-Off Distance Required to a height of 50 feet, (TODR), can become surprisingly high.

For example:

In still air, on a level dry hard runway at sea level with an ambient temperature of 10°C, an aeroplane requires a measured take-off distance to a height of 50 feet of 390m. This should be multiplied by the safety factor of 1.33 giving a TODR of 519m.

The same aeroplane in still air from a dry, short-grass strip (factor of 1.2) with a 2% uphill slope (factor of 1.1), 500 feet above sea-level (factor of 1.05) at 20°C (factor of 1.1), including the safety factor (factor of 1.33) will have TODR of:–

390 x 1.2 x 1.1 x 1.05 x 1.1 x 1.33 = 791m

You should always ensure that, after applying all the relevant factors, including the safety factor, the TODR does not exceed the take-off distance available. If it does, you must offload passengers, fuel or baggage. Better a disappointed passenger than a grieving widow! Do not rely on the 'It will be alright' syndrome.

Climb (and Go-around)
In order that the aeroplane climb performance does not fall below the prescribed minimum, some manuals/ handbooks quote take-off and landing weights that should not be exceeded at specific combinations of altitude and temperature ('WAT' limits). They are calculated using the pressure altitude and temperature at the relevant aerodrome.

Remember rate of climb decreases with altitude – don't allow yourself to get into a situation where the terrain outclimbs your aeroplane!

Landing

It is recommended that the Public Transport factor should be applied for all flights. For landing, this factor is x 1.43 (so that you should be able to landin 70% of the distance available).

Again when several factors are relevant, they must be multiplied. As in the takeoff case, the total distance required may seem surprisingly high.

You should always ensure that after applying all the relevant factors, including the safety factor, the Landing Distance Required (LDR) from a height of 50 feet does not exceed Landing Distance Available.

Engine failure: bear in mind the glide performance, miles per 1000 ft, of single-engined types and the ability to make a safe forced landing throughout the flight. Where possible, the cruise altitude should be selected accordingly.

Obstacles: it is essential to be aware of any obstacles likely to impede either the take-off or landing flight path and to ensure there is adequate performance available to clear them by a safe margin. AGA 3 section of the UK AIP includes obstacle data for a number of UK aerodromes. Excessive angles of bank shortly after take off, greatly reduces rate of climb.

Aerodrome distances: for many aerodromes information on available distances is published in the AGA section of the AIP or in one of the Flight Guides. At aerodromes where no published information exists, distances can be paced out. The pace length should be established accurately or assumed to be no more than 0.75 metres (21/2 ft). If you expect to use the strip frequently, it is better to measure the length accurately with the aid of a rope of known length.

Slopes can be calculated if surface elevation information is available, if not they should be estimated. For example, an altitude difference of 50 ft on a 750 metre (2,500 ft) strip indicates a 2% slope.

Be sure not to mix metres and feet in your calculation and remember, for instance, that a metre is more than a yard (see Conversion Table below).

Beware of intersection take-offs, displaced runway thresholds or soft ground which may reduce the available runway length to less than the published figures. Check NOTAMs, Local Notices etc.

Runway surface: operations from strips or aerodromes covered in snow, slush or extensive standing water are inadvisable and should not be attempted without first reading AIC 126/96 (Pink 131), 'Risks and Factors Associated with Operations on Runways Contaminated with Snow, Slush or Water'. A short wait could help in the case of standing water, hail, etc.

Advice: where doubt exists on the source of data to be used or its application in given circumstances, advice should be sought from the Flight Department, Safety Regulation Group, Civil Aviation Authority, Aviation House, South Area, Gatwick Airport, West Sussex RH6 0YR, Telephone (01293) 573113 Fax (01293) 573977.

Conversion Table:

1 kg	= 2.205 lb	1 lb	= 0.454 kg
1 inch	= 2.54 cm	1 cm	= 0.394 inches
1 ft	= 0.305 m	1 m	= 3.28 ft
1 US gall	= 3.785 litres	1 litre	= 0.264 US gall
1 Imp gall = 4.546 litres		1 litre	= 0.22 Imp gall
1 Imp gall = 1.205 US gall		1 US gall = 0.83 Imp gall

 

FACTORS MUST BE MULTIPLIED i.e. 1.2 x 1.3
	TAKE-OFF	LANDING
CONDITION	INCREASE IN DISTANCE FACTOR TO HEIGHT 50 FEET	INCREASE IN LANDING FACTOR DISTANCE FROM 50 FEET
A 10% increase in aeroplane weight, e.g. another passenger	20% 1.2	10% 1.1
An increase of 1,000 ft in aerodrome elevation	10% 1.1	5% 1.05
An increase of 10°C in ambient temperature	10% 1.1	5% 1.05
Dry grass* – Up to 20 cm (8 in) (on firm soil)	20% 1.2	20% + 1.2
Wet grass* – Up to 20 cm (8 in) (on firm soil)	30% 1.3	30% + 1.3 When the grass is very short, the surface may be slippery and distances may increase by up to 60%.
A 2% slope*	uphill 10% 1.1	downhill 10% 1.1
A tailwind component of 10% of lift-off speed	20% 1.2	20% 1.2
Soft ground or snow*	25% 1.25 or more	25% + 1.25 or more
NOW USE ADDITIONAL SAFETY FACTORS (if data is unfactored)	1.33	1.43

Notes: 1. * Effect on Ground Run/Roll will be greater.

1. + For a few types of aeroplane e.g. those without brakes, grass surfaces may decrease the landing roll. However, to be on the safe side, assume the INCREASE shown until you are thoroughly conversant with the aeroplane type.

2. Any deviation from normal operating techniques is likely to result in an increased distance.

So, if the distance required exceeds the distance available, changes will HAVE to be made.