Performance Flashcards
Gross performance vs measured performance
“Performance” here means any data point regarding an aircraft’s performance.
Gross performance is the expected mean for the fleet, measured performance reflects the pre-production aircraft and is expected to be better.
Gross performance is estimated from measured performance by “factoring”.
Acceptable performance criteria
One in a million chance of commercial air transport craft being in a situation as a result of failure to achieve required performance standard.
This is called “net performance”.
Difference between net performance and gross performance
This is our safety margin. It includes the fact that performance of an individual aircraft is somewhere in a normal curve, whilst the gross performance reflects the mean.
Net performance is what we use in calculations to achieve required safety margins.
Impact of event likelihood on safety margins
The 1 in a million standard is a combination of likelihood of being in a situation and failure to meet performance standard. So unlikely events (engine failure) result in lower safety margins than likely events (full engine climb).
Events with less than 1 in a million chance (e.g. 2 of 4 engines failing) have no safety margin.
Commercial vs private performance figures
CS25 aircraft are all intended for public use so published figures are net performance.
CS23 will generally be gross performance (50:50 chance of being worse or better than this!). Must be adjusted to net performance for public use, whilst this is advisory for private flight.
Performance Classes
Clearway requirements
- shape
- width
- max slope
- obstacles
Rectangular
>500ft wide
Upward slope <= 1.25%
No protruding object or terrain, except lights if 26in or less above the runway and to each side of it.
TODA
TORA + Clearway
ASDA
TORA + Stopway
Balanced field
TODA = ASDA
(i.e. stopway = clearway)
Balanced field take off
- description
- effect of added clearway/stopway
TODR = ASDR [note: required]
Maximises mass by using choosing a V1 that balances ASD and TOD requirements.
Extra clearway allows lower V1 (longer at OEI speed) to increase mass.
Extra stopway allows higher V1 (longer stopping distance) to increase mass.
Amendments to TODA
- Class A
Max TODA = 1.5 x TORA
So clearway over 50% of runway length can’t be fully used.
Amendments to TODA
- Class B
If no stopway or clearway, can only use TORA / 1.25.
With a stopway or clearway minimum of:
i) TORA
ii) TODA / 1.15
iii) ASDA / 1.3
Line-up distance adjustments
May be required for class A aircraft where there isn’t sufficient take off threshold or turning apron.
Accommodates a 90 degree or 180 degree turn (as required).
[ASDA adjusted to nosewheel, TODA to main gear]
Declared temperature
Average monthly temperature plus half the ISA deviation. Used for airlines in scheduling landings for performance calculations.
Flight path angle designations
We assume wing chord in line with longitudinal axis.
Correction for density altitude from pressure altitude
120ft per degree C (difference to ISA)
Rated Thrust
Maximum thrust levels:
i) Maximum T/O thrust
ii) Maximum continuous thrust
iii) Maximum go-around thrust
[Other limits such as climb thrust exist, but these are not rated thrusts]
Jet thrust vs speed
2 opposing factors - ram air effect at high speed increases mass of air and thus thrust, but increased drag reduces momentum of the air to offset this.
Traditional/standard low-bypass jets have a small thrust decrease to Mn 0.4 then recover. High bypass don’t benefit much from ram effect so thrust keeps decreasing with speed.
T/O thrust vs TOGA thrust
Difference is due to speed, take-off thrust is set at 40 to 80kt, TOGA is set at maybe 150 to 170kt.
Thrust limitations (jet)
Limited by Turbine Gas Temperature (TGT) and internal pressure in the engine.
Internal pressure limitation is constant, but TGT is dependent on OAT (the engine adds a certain amount of heat to the existing air temp).
Thus thrust is constant (pressure limited) up to a certain temperature, then decreases (TGT limited).
Jet power
Jet power = thrust x TAS
Generally thrust is constant so get a straight line relationship with TAS.
Fixed pitch propeller thrust with speed
Angle of attack of blade decreases as speed increases so thrust decreases with speed.
Propeller power
Power = thrust x TAS (same as jet)
However as thrust decreases with speed, get an “n” shaped chart with speed, where initially it increases from zero, then at some point starts to decrease.
Critical Engine
The engine whose failure would have the biggest impact on the aircraft performance.
On 4 engine will be one of the outer ones.
On 2 jet engine probably don’t have one.
On 2 prop will have one due to asymmetric blade effect.
Asymmetric blade effect
At high AoA the prop is “leaning back” so blade travels further going down than when going up. This moves thrust (at high AoA) to the right (for clockwise props) of centre. This makes the left engine the critical engine (its failure gives biggest asymmetry).
Speed stability on approach
- Situation
- Impact of flaps
Especially for jets, speed is below V(MD) therefore speed unstable.
Flaps increase drag (higher parasite drag) whilst maintaining lift (constant induced drag). Resulting drag curve shifts up and to the left, meaning V(MD) is lower and flight more stable.
So flight is less efficient, but stability is better (plus flaps can quickly be put away for a go around).
Effect of decreasing mass on speed stability V(MD)
Parasite drag stays the same, but induced drag reduces as mass decreases.
Drag curve moves down and to the left, increasing stability on approach.
Which speed is used to express V(MD)?
EAS - At low speeds (i.e. around V(MD)) compressibility is negligible so this is effectively same as IAS or CAS
Maximum speed
- How it is determined
- Effect of altitude (in EAS and TAS)
Determined by intersection of thrust available with the drag curve. As speed increases thrust increases a bit, but induced drag increases a lot until speed is capped.
However thrust reduces with altitude so maximum EAS reduces. At the same time TAS is increasing (relative to EAS) so hard to say the impact of altitude on maximum TAS.
Speeds in 1/2 rho V^2
V is TAS
However the total dynamic pressure element of 1/2 rho V^2 relates to CAS/EAS.
Stall speeds
- V(S1g)
- V(SR)
- V(S)
V(S1g) is at the critical angle of attack, point of maximum lift. This is when the wing STARTS to stall.
V(SR) is the stall reference speed, which is taken to be equal to V(S1g).
V(S) is the minimum speed at which aircraft is controllable, so lower than V(SR). Used to be based on 0.94 x V(SR).
Which speed is used to asses power required, V(MP) etc?
TAS
This is because power is defined as thrust x TAS.
Power required curve
- description
This is the drag curve (parasite & induced) multiplied by TAS.
Gives Parasite power, induced power and V(MP).
Power required curve
- diagram
V(MD) on the power required curve
As power required curve is drag curve x TAS, V(MD) is found by drawing a tangent through (0,0) to the curve. Will be at a higher speed than V(MP).
Effect of increasing altitude on drag/power required curves
Density altitude doesn’t directly affect drag (induced or parasite). If IAS is constant, drag will be the same.
So drag curve (plotted against V, TAS) just moves to the right with increasing altitude (same drag at higher TAS).
Power required is drag x TAS so that curve slides up and right along the same tangent.
Take off run (TOR)
Take off distance (TOD)
Take off run includes the ground run and PART (usually half) of the airborne segment to screen height.
Take off distance includes the whole airborne segment to full screen height.
V(R)
V(LOF)
V(2)
V(R) = Speed at which you rotate
V(LOF) = Lift off speed, slightly higher than V(R)
V(2) = Speed at the screen height (end of TOD) which will be higher still [aka Take off safety speed]
Chart of thrust vs airspeed (including take off range) for prop, high bypass jet and low bypass jet
Explanation of thrust vs airspeed for jets
All jets experience reduction in thrust during takeoff run due to intake momentum drag (air hitting front of engine).
At higher speeds increased ram effect acts to increase thrust. In low bypass this is enough to start to increase thrust. In high bypass thrust still reduces to zero (beyond mach 1.0).
Chart of drag and thrust against speed during take off run
Elements of drag during take off run
Wheel drag: Decreases linearly as speed increases due to lift gradually reducing downforce on the wheels.
Aerodynamic drag: Usual drag components, parasite & induced. Parasite increases with square of speed, whilst induced is low until V(R) then suddenly increases. This leads to a rise in total drag at V(R).
Impact of increase in V(R) on takeoff run
Obviously higher V(R) means longer takeoff run.
Effect is exponential. This is because thrust decreases with speed, whilst aerodynamic drag is increasing. The difference is the net force acting to accelerate the aircraft, which is reducing. So acceleration reduces along the runway and if you need a bigger V(R), you need a lot more runway.
Even with constant acceleration, TOR increases with square of V(R).
V(R) requirements for:
- Single engine (class B)
- Multi engine (class B)
- Class A
Single engine (B): V(R) >= V(S1)
Multi engine (B): V(R) >= 1.1 x V(S1) & 1.05 x V(MCa)
Class A: V(R) >= V(MU) + small margin
[V(MU) = minimum unstick speed]
Tyre speed limitations
- point of greatest tyre speed
- 2 key factors affecting speed limit
As V(R) is an airspeed, tyre speed limitation can become a separate issue.
2 key factors are:
- temperature &
- rotation rate of tyre.
NOT pressure
Using headwind and tailwind in take off distance calculations
Use 150% of tailwind and 50% of headwind for margin of safety.
Effect of flap setting on take off
Flap increases lift and drag.
Initial stages have limited drag impact, but extra lift allows earlier rotation and shorter TOR.
Later stages have more drag impact and thus increase TOR.
After take off flap is inefficient and best climb gradient is with clean config.
[Generally exam refers to flap reducing TOR]
Chart showing impact of flap setting on climb gradient and field length
V(stop) and V(go)
V(stop) is the speed above which the aircraft can’t be stopped within the ASDA.
V(go) is the speed below which (based on engine failure) rotation at V(R) and reaching V(2) at 35ft screen height is not possible.
Both vary with mass and are probabilistic lines (i.e. 50:50 chance of making it).
V(stop) and V(go) plotted against mass
One Engine Inoperative
Field Length Limited
Take Off Mass
(OEI FLL TOM)
OEI FLL TOM is where V(go) = V(stop) = V(1) - called the decision speed
i.e. highest mass at which there is no “die” portion of the V(stop) & V(go) chart!
Effect of increase in ASDA on V(1) and OEI FLL TOM
Increasing ASDA affects V(stop), the extra length means we can safely stop from a higher speed.
The red line shifts up and to the right so V(1) and OEI FLL TOM increase.
Effect of increase in clearway on V(1) and OEI FLL TOM
Increase in TODA reduces the speed required to achieve V(R) and screen height at V(2).
So green curve moves down and to the right, causing V(1) to decrease and OEI FLL TOM to increase.
Chart showing effect of clearway increase on V(1) and OEI FLL TOM
V(EF)
Speed at which an engine failure is assumed to occur. CS25 allows a minimum 1 second delay between V(EF) and V(1) to account for startle effect.
Referred to as “recognise and REACT” in exam, although react doesn’t include starting braking.
Action on emergency stop at V(1)
Must call “Stop” at V(1) at the very latest, after which it is assumed braking commences within 2 seconds.
NOTE: In exams assume “action taken” at V(1), the 2 second delay is for deceleration to commence (i.e. aircraft to respond to inputs).
Changes for wet runway takeoff
V(EF) is reduced by 10kt, with correspondingly lower V(1) speed.
We are further from V(R) at this point so TOD will increase. Therefore screen height is reduced to 15ft (for OEI only!), with V(2) only required at the eventual 35ft point.
TO performance calculations for wet runways - process
Need to calculate for wet and dry and use worst case scenario.
TODA: Wet runway with 15ft screen (and slower V(1)) may or may not be more limiting than dry with 35ft.
ASDA: Calculate for wet and dry (with respective V(1wet) and V(1dry)) and also for all engines & engine out. Use the most limiting length.
Reverse thrust and ASD required (ASDR)
Reverse thrust can only be factored in for wet runway stopping, not dry runways
Impact of runway slope on ASDR
Two effects.
Downwards slope increases stopping distance, but also assists acceleration towards V(R).
The benefit to acceleration is the greater effect so downslope reduces ASDR.
Safety factors in take off distances required
We use gross figures (i.e. 50:50 chance of success) rather than net, as the probability of engine failure at exactly V(EF) is far less than 1 in 1mn.
V(MCG)
Minimum control speed on the ground. Below this speed rudder effectiveness in the case of engine failure, is insufficient to maintain centreline (maximum 30ft deviation allowed).
Assumptions used to establish V(MCG)
Based on:
- Multiple (or worst) take off configuration
- Maximum thrust on operating engines
- Worst possible CoG
- Worst possible take off weight
- Take off trim
They do NOT take crosswind into account (due to remote probability).
Effect of V(MCG) on V(1)
Obviously V(1) must be above V(MCG) otherwise continuing take off at V(1) would risk going off runway.
If V(MCG) falls within the range of available V(1)s due to V(stop) and V(go), just choose a V(1) above V(MCG).
If it doesn’t, V(MCG) will be a limiting factor for TOM. Max TOM will be where V(MCG) intersects the V(stop) line.
V(MCA) or V(MC)
- description
- what it limits
Minimum speed for control in the air.
At V(MCA) up to 5 degrees of bank can be used along with rudder to keep aircraft straight (OEI), NOT maintain altitude.
However this relates to V(LOF) instead of V(R), and also we can’t get 5 degrees of bank just after takeoff.
So in practice we require:
V(R) > 1.05 V(MCA)
Effect of conditions on V(MCG) and V(MCA)
Primarily concerned with the level of thrust from the operating engine.
High air density will mean increased thrust, higher yawing forces to be offset by rudder and therefore higher airspeed needed to allow rudder to function.
V(MBE)
Maximum braking energy speed (i.e. maximum speed where brakes can cope with the energy to be dissipated).
V(1) must be lower than V(MBE).
Similarly to V(MCG), this could be a limiting factor to TOW if V(MBE) doesn’t fall in available V(1) range.
Effect of conditions on V(MBE)
Increase in V(MBE) caused by:
- High temperature
- High pressure altitude
- Tailwind
- Downward slope
Combination TOM limitation chart
TOM limitation chart
- Distance vs V1
V(1) is the time engine failure is responded to, so lower V(1) results in longer distance with one engine and longer TOD/TOR
Diagram of forces in a climb
[note life & weight direction]
Effect on V(x) of temperature and pressure
Thrust reduces with lower air density but drag (relative to EAS) stays the same. V(x) unchanged, but excess thrust lower so angle of climb achieved is lower.
Effect of mass and flaps on V(x) - chart
Summary of effect on V(x) for jet & prop
- altitude
- temperature
- mass
- flap
Chart of V(Y) (TAS vs power)
At sea level (TAS = IAS = EAS)
V(X) and V(Y) on power chart
As altitude increases (or temp +) power available falls and power required slides up. At aircraft ceiling the power available line will be a tangent to power required and V(x) = V(y) = V(MD).
Summary of effect on V(y) for jet & prop
- altitude
- temperature
- mass
- flap
