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know the performance of your aircraft
abridged from GASCO
introduction
Accidents such as failure to get airborne, collision with
obstacles after take-off and over-run on landing, occur frequently to
light aeroplanes – (over 20 cases per year). Many have happened at short
strips, often when operating out of wind or where there was a slope. Poor
surfaces such as long or wet grass, mud or snow, were often contributory
factors. These were performance accidents and many, if not all, of these
accidents could have been avoided if the pilots had been fully aware of
the performance limitations of their aeroplanes.
The pilot in command has a legal obligation to check that the aeroplane
will have adequate performance for the proposed flight. If you are using
a 3000 metre runway a cursory check of performance will do, but where is
the dividing line – 700, 1000 or 1500 metres? This will be decided by a
large number of variables and only by reference to performance data,
including climb performance, can the safety, or otherwise, of the
particular flight be properly determined.
finding the information
The data needed to predict the performance in the expected conditions may
be in any one of the following:
• The 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 Change Sheets
and/or Supplements giving additional performance data which may either
supplement or override data in the main document, e.g. a ’fleet
downgrade‘.
• For some imported aeroplanes, an English language Flight Manual
approved by the airworthiness authority in the country of origin, with a
supplement containing the performance data approved by your aviation
authority.
use of performance data
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. It is unlikely that your aviation authority has
verified the Performance Data on all foreign aeroplanes; in some cases a
single spot check may be made.
To ensure a high level of
safety on UK Public Transport flights, there is a legal requirement to
apply specified safety factors to un-factored 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 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. Manuals usually make it clear if factors are
included but if in any doubt you should consult your aviation authority.
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 local
limitation contained in a Supplement which is not referred to in the text
of the main document.)
If any advice/information given here 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.
performance planning
A list of variables affecting performance together with Factors for
non-Public Transport operations are shown in tabular form at the end of
this article. 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 leaflet and use
this where other Information is not available.
general points
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 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, particularly those with laminar flow aerofoils. 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, as will wheel fairings jammed
full of mud, grass, slush, etc.
Engine failure: since an engine failure or power loss (even on
some twin-engined aircraft) may result in a forced landing, this must be
borne in mind during all stages of the flight.
Performance during aerobatics: remember that variations in
aeroplane weight will directly affect its performance during aerobatics
(even, for example, steep turns) and outside air temperature/ altitude
will similarly affect engine power available. Hot day aerobatics in a
heavier than normal aeroplane require careful planning and thought.
take-off points to note
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.
Twin engines: on
twin engined aircraft, if there is an engine failure after lift off, you
may not reach the scheduled single engine rate of climb until:
• the landing gear and flaps have retracted (there may be a temporary
degradation as the gear doors open)
• the best single engine climb speed, ‘blue line speed’, has been
achieved.
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.
KEEP IN ASYMMETRIC PRACTICE
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. An uphill slope reduces acceleration.
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.
Abandoned take-off: Many multi-engined aeroplane manuals include
data on rejected take- off distances. Some aircraft quote a minimum
engine rpm that should be available during the take off run.
Engine power: check early in the take off run that engine(s)
rpm/manifold pressure are correct. If they are low, abandon take off when
there is plenty of room to stop. Brief use of carb heat at the hold
should ensure carb ice is not forming.
landing points to note
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 a wet surface, 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.
safety factors
Take-off
It is strongly recommended that the appropriate Public Transport factor,
or one corresponding to that requirement, 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 leaflet gives guidance for pilots of
aeroplanes for which there is only UNFACTORED data.
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 land in 70% of the distance available).
Again when several factors are relevant, they must be multiplied. As with
take-off, 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.
additional information
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. Excessive
angles of bank shortly after take off greatly reduce rate of climb.
Aerodrome distances:
for many aerodromes information on available distances is published 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 (2 ½ft). 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.
A short wait could help in the case of standing water, hail, etc.
Notes:
1. * Effect on Ground Run/ Roll will be greater.
2. + 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.
3. Any deviation from normal operating techniques is likely to result in
an increased distance.
If the distance required exceeds the distance available, changes will
HAVE to be made.
FACTORS MUST BE MULTIPLIED i.e. 1.20 x 1.35
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