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 levels
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 on approach
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 parasite 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.
Highest at V(LOF).
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
All engine TODR assessment
- Safety factor
- V(3)
As all engine takeoff is very likely a safety factor of 1.15 is applied to TODR.
V(3) is the speed at 35ft screen height with all engines, simply defined as being higher than V(2).
This TODR is unlikely to be limiting factor for 2 engine, but for 4 engine aircraft the 1.15 factor could have a bigger impact than the loss of one engine.
All Engine Operating
Field Length Limited
Take Off Mass
(AEO FLL TOM)
This is the TOM restriction based on all engine calculations (with 1.15 safety factor).
In typical case this is a vertical line on the V(stop)/V(go) chart which will be higher than the OEI FLL TOM intersection with V1.
If lower (e.g. 4 engine) it will be to the left of the V1 intersection. V1 still valid but that TOM can’t be achieved.
TORA vs TODA limitations
Need to get to half of the screen height by end of TORA.
With no clearway TODA will be the limitation, but as clearway increases TORR becomes a limiting factor.
In reality manufacturers charts use simplifying assumptions limiting assumed clearway size (NOT the same as the 50% legal limit) such that TODA remains the limiting factor. In the exam we have to consider both TODR and TORR.
TORR
The longest of:
OEI dry: TOR + half distance to 35ft
[OEI wet: TOR + distance to 15ft, if wet!]
AEO: 1.15 x (TOR + half distance to 35ft)
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 lift & weight direction]
Formula for climb gradient
Climb Gradient = (T - D) / Weight
[approximation from balance of forces:
T = D + W x sin(theta)]
[Can assume lift = weight at low angles of climb?]
V(x)
- definition
- prop vs jet
V(x) is best angle of climb speed.
Found by best excess thrust.
On prop, thrust decreases with speed so V(x) is at a low speed close to the stall.
On jet thrust is more steady so V(x) closer to V(MD), a higher speed.
Effect on V(x) of temperature and pressure (jet)
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) - description
Mass and flaps affect drag rather than thrust.
Mass increases induced drag due to higher required lift (at given speed), so left side of drag curve shifts up.
Flaps increase parasite drag (lift maintained for given speed by adjusting attitude) so right side of drag curve shifts up.
So V(x) is higher for extra mass, lower for extra flaps (see chart).
Effect of mass and flaps on V(x) - chart
Summary of effect on V(x) for jet & prop
- altitude
- temperature
- mass
- flap
Climb gradient vs flight path angle
Climb gradient (which we focus on) is an air gradient, so not affected by wind.
Flight path angle is in terms of ground distance, so need to adjust climb gradient for wind.
V(2) minimums
- class B
Max of:
- V(MC) [ 1.1 x V(MC) for twin]
- 1.2 x V(S1)
- A safe speed under reasonable conditions
V(2) minimums
- class A
Max of
- 1.1 x V(MC)
- 1.13 x V(SR)
When are V(MC) or V(SR) expected to restrict V(2min)?
V(MC) is highest at high density (low altitude, low temp) due to higher thrust. So at low altitude V(MC) likely to limit, then V(SR) at higher altitude.
Nature of V(2) minimum restrictions with altitude
V(MC) is basically fixed with altitude (and mass).
V(SR) increases with altitude (and mass) therefore 1.13xV(SR) is likely to be the limiting factor at higher altitudes, 1.1xV(MC) at lower altitudes.
With higher mass V(SR) takes over earlier.
Actual initial climb speed for jets
Usually aim for V(2) + 10kt.
Whilst V(2) is close to V(x) for prop, for jets V(X) is higher, so V(2) + 10kt is more efficient.
Also, V(2) + 10kt is closer to V(EF) so if an engine fails the set up is safer.
Take off climb limit
- relevant config
- required gradients
There is a minimum requirement for AIR climb gradient, based on gear up and OEI.
2 engines: 2.4%
3 engines: 2.7%
4 engines: 3.0%
Combined with altitude & temp data this produces a mass limitation, to be considered along with field length take off mass limits.
Rate of climb formula
ROC = TAS x (T - D) / weight [gradient]
= (power available - power required) / weight
Chart of V(Y) (power vs TAS)
- jet and prop
At sea level (TAS = IAS = EAS)
Effect of mass and flaps on V(Y)
Similar to V(X), but impact on power required looks different to impact on drag.
Maximum rate of climb will decrease, V(Y) increases with higher mass (more induced drag), V(Y) reduces with flaps (more parasite drag).
V(X) and V(Y) on power chart (jet)
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).
V(Y) with increasing altitude and temp
- TAS and IAS
Thrust reduces, so the power available line pivots downwards.
Drag not affected but increase in TAS causes power required to slide up and right.
So V(Y) increases in terms of TAS, however in terms of IAS it approaches V(X) [which is lower] due to the convergence of V(X), V(Y) and V(MD) as the two power lines converge and ceiling is approached.
Summary of effect on V(y) for jet & prop
- altitude
- temperature
- mass
- flap
V(XSE) and V(YSE)
V(X) and V(Y) with single engine (OEI).
Tend to be close to V(X) and V(Y), especially for jet.
If pushed in exam they are inbetween V(X) and V(Y) so:
V(X) < V(XSE) < V(YSE) < V(Y)
Climb speed with engine out
Accelerate past V(EF) to V(2) and climb at V(2) until at least 400ft AAL, when flaps can be retracted.
Climb speeds with all engines (in reality, to ToC)
Speed will be limited to 250kt EAS up to 10,000ft so maintain that speed up to FL100. TAS will be increasing with altitude.
Accelerate at FL100 to a set speed (perhaps 300kt) and hold that until crossover altitude, where mach limit is reached.
Follow mach limit from then.
Effect of cost index on climb
High cost index implies low fuel cost.
This would mean adopting fast speeds in general (subject to limits, e.g. 250kt <FL100). Climb gradients shallower. Top of climb reached later, but at a further ground distance so trip time reduces.
Optimal cruise speeds
- Endurance
- Range
Endurance - V(MD)
Range - 1.32 x V(MD) [tangent to drag curve that gives V(MD)
Step climb (cruise)
We want optimal speed (1.32 x V(MD)) and also optimal thrust (c. 90%).
These coincide at a given altitude, based on mass.
Thus we choose altitude to get optimal speed and thrust, but this altitude increases as fuel burns off. So we use a step profile, changing FL as we burn off fuel.
Long range cruise
4% faster than best range speed, but with 99% of the range.
Buffet boundary limit
Restricted on slow side by stall speed and high side by mach limit.
As altitude increases these close in together, creating “coffin corner”.
Increased mass brings both in (mostly the stall speed), thus reducing maximum altitude.
Buffet boundary limit
- Load factor
Increased load above 1g (e.g. due to turbulence) brings in the two limits. Thus buffet boundary charts usually based on 1.3g to give some margin for safety.
Need awareness that increased turbulence could take you outside safe operating conditions.
Specific fuel consumption
- Jet
- Prop
Jet: Fuel flow / thrust
Prop: Fuel flow / power
Optimal speeds for prop
As SFC is based on power, we have max endurance speed of V(MP) instead of V(MD).
Best range speed is then a gradient to the power curve, which gives V(MD) [rather than a multiple of V(MP) as with jets]
Optimal cruising altitude in prop
As with jets, there is another target which in this case is 100% open throttle.
This would be faster than V(MP) at ground level so get more efficient flight at around 5000 to 7000ft where full throttle will coincide with V(MP).
Optimal altitude relates to best range or endurance?
Relates to best range.
Best glide angle
- How to calculate it
- Speed
- Effect of mass
Glide angle = C(D)/C(L)
Speed = V(MD)
C(D)/C(L) is independent of mass, thus so is descent angle.
However V(MD) increases with mass so heavier aircraft will travel faster for best angle and thus RoD is higher.
Power on descent angle
sin(theta) = (D - T) / weight
Reversal of climb angle formula because now thrust is less than drag.
Rate of descent formula
ROD = (power required - power available) / weight
Reversal of rate of climb formula
Glide endurance speed - v(?)
This is the speed at which rate of descent is minimised. Weight is fixed so we need to minimise power required.
Thus V(MP).
This is less than the best glide range speed V(MD).
Emergency descent
Need maximum speed and maximum drag.
Flaps (etc) increase drag but may limit speed.
Thrust likely to be idle as speed will be too high otherwise.
Top of Descent rules of thumb
- Air distance per 1000ft
- Adjustment for wind
3nm per 1000ft in jet
Modify distance by (windspeed/10)%
e.g. 50kt headwind, reduce distance by 5%
Effect of cost index on descent
Typical descent uses speedbrakes to get rid of energy. High fuel cost (low cost index) will mean an earlier descent, using potential energy to reach the destination and take longer to get there.
Landing restriction configurations
- Missed Approach Climb (or approach climb)
Go-around from above DH with OEI.
- Go-around thrust on remaining engines
- Landing gear up
- Approach flap set
Assumptions for missed approach climb
- Climb speed limit
- Climb gradients
- Bank towards operating engine
Climb speed <= 1.4 x V(SR)
Climb gradient:
- 2 engines: 2.1%
Bank up to 2 to 3 degrees towards operating engine
Missed approach gradient AEO
- Class A
Class A: 3.2%
Landing restriction configurations
- Baulked Landing Climb (or landing climb)
Go-around from below DH with AEO.
- Go-around thrust all engines
- Landing gear down
- Landing flap set
V(MCL)
- required manoeuvrability
Minimum control speed (landing)
Requires:
- ability to maintain straight & level flight with up to 5 degrees bank towards live engine.
- ability to roll away from failed engine by 20 degrees within 5 seconds from straight flight
[In effect means you have control in case of critical engine failure in go-around]
Landing distance required
- description
- 2 components
From screen height (typically 50ft but could be 35ft for steep approaches) above threshold to the point when you stop.
Split into landing airborne distance and landing ground run.
V(REF)
- description
- value (class B and A)
Landing reference speed.
This is your speed at screen height.
Will usually be V(REF) + 10kt on approach, V(REF) - 10kt at touchdown.
V(REF) = 1.3 x V(S0) class B
V(REF) = 1.23 x V(SRO) class A
Not less than V(MCL)
Impact of failure on LDR
- anti skid brakes
- reverse thrust
Anti-skid: +50%
Reverse Thrust: +10%
[Much higher for prop aircraft]
Impact of mass on landing distance (2)
2 effects
- Increases speed as it impacts stall speed and thus V(REF)
- Increased mass directly increases kinetic energy (1/2 M V^2)
Runway landing distance factors
- wet (class A & B)
- grass on firm soil (B only)
- short wet grass (B only)
Wet: 1.15
Grass w firm soil: 1.15
Short wet grass: 1.6
[Calculated for WHOLE landing distance, even though it only affects the ground run]
Landing distance safety margin
Jet: Land within 60% of LDA
Runway landing distance factor for downslope
5% extra for every 1% downslope.
Class A only need to calculate for 2% or higher downslope
Obstacle clearance
- Take off obstacle domain
[with track changes above or below 15 degrees]
Half-width starting from end of TDA is:
Wingspan <=60m: 60m + 0.5 x WS
Wingspan >60m: 90m
Then increases by distance / 8 up to max 300m half width [600m if track changes above 15 degrees]
Obstacle clearance
- Class A Net Take Off Flight Path (NTOFP)
For performance calculation purposes we consider engine failure at V(EF).
i) End of TODR (35ft) to gear up point, flown at V(2)
ii) From gear up to level off height, also V(2) with take off thrust
iii) Level segment for flap retraction, ends when in clean config. Accelerate to final segment climbing speed and reduce thrust on remaining engine to max continuous (MCT)
iv) Climb to at least 1500ft, at a climb speed (eg V(X)).
V(FTO)
- description
- minimum value
Class A speed to be reached at end of acceleration phase of take off (3rd phase).
>= 1.18 V(SR) and “sufficient to provide the required climb gradient”
Obstacle clearance
- Class A NTOFP acceleration height
i.e. height of 3rd phase
Minimum of 400ft
Can be limited by maximum full thrust time (around 10 minutes)
Typically major obstacles are cleared in phase 2 so don’t impact the acceleration height and a standard one can be chosen.
Obstacle clearance
- Class A Limitations on turns during take off
NOT below 50ft clearance from obstacle
Bank angle normally limited to 15deg up to 400ft and 25deg after.
Can get approval for 20deg between 200 and 400ft and 30deg after.
[50ft clearance required when turning]
Obstacle clearance (class A)
- Minimum climb gradients phase 2
- Clearance required to the net path
Twin: 2.4% gross (1.6% net)
Triple: 2.7% gross (1.8% net)
Quad: 3.0% gross (2.0% net)
35ft clearance required
Obstacle clearance
- Minimum available climb gradients in phase 4 (class A)
Twin: 1.2% net
Triple: 1.5% net
Quad: 1.7% net
Obstacle clearance
- En-route obstacle clearance (Class A)
1000ft above terrain within 5nm of route while maintaining height
2000ft during driftdown stage
This is about dealing with engine loss in flight. You would generally use MCT for as long as possible to maintain altitude and diagnose, before driftdown to a level where height can be maintained.
Can assume fuel dumping to help with driftdown.
Obstacle clearance
- En-route phase definition
From 1500ft above departure to 1500ft above destination
Obstacle clearance
- Net vs gross gradient (gliding)
Net gradient = gross gradient + 0.5%
0.5% is the safety factor so obstacle clearance requires us to clear obstacles even with a 0.5% steeper gradient
Regulated Take Off Weight (RTOW) [or mass]
Table for a given runway that calculates the most restrictive of all take off masses:
- FLL TOM
- Climb Limited TOM
- Obstacle Limited TOM
- Tyre Speed Limited TOM
- Maximum Structural TOM
Purpose of reduced thrust take off
Prolongs engine life
NOT to reduce fuel consumption or noise, does the opposite!
Flex take off
If TOW is less than MTOW, can use an assumed OAT (greater than actual) in FMS. FMS will use a lower fuel rate based on the higher assumed ambient temperature, reducing thrust and OEI limitations will just be achieved.
Regulatory thrust reduction limits
Limit on how much you can reduce thrust for flex takeoff - 25%.
DOES NOT affect derated thrust.
Temperatures for flex take off
T(REF) is the temperature where thrust is TGT limited, relates to the MTOW.
T(FLEX MAX) is the temperature relating to thrust reduction limit [max temperature you can select]
T(FLEX) is the chosen flex temperature in between these two that you use to take off.
V(MCG) and V(MCA) for flex take-off
Because full thrust is available even on flex take-off, V(MCG) and V(MCA) must be calculated based on full thrust.
Otherwise there is a risk of failure to control aircraft if full thrust is applied.
Conditions when flex take off is not allowed
- Icy (or very slippery) runways
- Contaminated runways
- Anti-skid unserviceable
- Reverse thrust unserviceable
- Increased V(2) procedure
- Power Management Computer off (MRJT1)
Conditions when flex take off MAY not be allowed
Wet runways require suitable performance accountability (which in practice is the case for most operators). So wet runways ARE allowed.
Not recommended if windshear expected after takeoff.
Derated thrust take-off
- description
- possible on contaminated runways?
AFM must contain a complete set of performance calculations based on a limited thrust setting for the engines. This derated thrust setting can then be selected in FMS.
Unlike flex take-off this is considered to be a “normal” take-off, so no restrictions for contaminated runways (etc.)
V(MCG) and V(MCA) for derated thrust take-offs
For derated take-offs they will be reduced based on the lower available thrust.
This does however mean that TOGA thrust CANNOT be selected during derated thrust take-off. Likely to be restricted until flaps fully retracted.
Effect of derated thrust on V(MCG) limited take-off
If the take-off is V(MCG) limited, derating thrust will increase the MTOW.
Likely to see increasing MTOW with initial stages of derating, then eventually increasing again with higher derating levels.
Increasing V(2) to overcome climb limit
Climb limit exists because of limited climb gradient at V(2min) which is less than V(X).
Increasing V(2) therefore helps against climb limit at the expense of extra runway. If the runway is available this can be used to increase a climb limited MTOM.
NOTE: Extra speed to V(R) will affect tyre limits (etc.) so these need to be checked.
Increasing V(2) to improve climb gradient - which sectors are improved?
Using higher V(2) to help with obstacle limit involves trading some of the capacity gained for more weight and retaining some for better gradient (unlike climb limit which purely helps with weights).
Doesn’t help with VERY close obstacles (as TOR is longer) or those in final segment (where speed is V(X) anyway).
Helps with SECOND SEGMENT.
Contaminated runway
Significant portion (used to be 25%, now undefined) of the length and width of runway BEING USED, covered with one of:
- Standing water (>3mm)
- Slush
- Wet snow (snowball)
- Dry snow (no snowball)
- Compacted snow
- Ice
- Specially prepared winter runway (sand, grid, mechanical or chemical treatment)
Non-contaminated runway surfaces
Dry
Damp (not reflective)
Wet (water, slush or snow <3mm, or reflective but no standing water)
Effect of contaminated runway on friction & drag
All types reduce friction.
Some increase drag (good for RTO, bad for TO):
Standing water & slush up to 15mm
Wet snow 5-30mm
Dry snow 10-130mm
Rest have no drag impact:
Wet snow <5mm
Dry snow <10mm
Compacted snow
Ice
Specially prepared winter runway
Contaminated runway drag:
- displacement
- impingement
Displacement is drag due to having to push the contamination out of the way
Impingement drag is due to the contamination hitting the fuselage.
Effect of contaminated runway on performance calcs (take-off)
MTOM reduces
Speeds (V(1), V(R), V(2) decrease)
Effect of contaminated runway on:
- LDR
- TODR
- ASDR
All distances increase EXCEPT:
TODR is unchanged for ice & compacted snow as there is no drag effect and friction effect isn’t a problem as there is no braking on take off.
Dynamic hydroplaning speed calc
9 x sqrt(psi)
7.7 for static wheel, but this is likely to be viscous not dynamic hydroplaning
Viscous hydroplaning
- description
- speed calc
When a thin film of liquid prevents the tyre contacting the runway, LIKELY AT RUNWAY THRESHOLDS.
Speed = 7.7 x sqrt(psi)
Reverted rubber hydroplaning
Can happen after a skid where high temperature in tyre boils and thin layer of water and vaporised rubber to steam.
Braking action report format
Good: 5
Good to medium: 4
Medium: 3
Medium to poor: 2
Poor: 1
Less than poor: 0
Which part of ops manual has performance info?
Part B
Pavement Classification Number format
eg 50/R/B/X/U
50 - PCN number
R - (R)igid or (F)lexible [need right one]
Ignore the other items!
Occasionally exceeding PCN
Up to 5% of annual aircraft movements can be:
Rigid: Up to 5% higher than PCN
Flexible: Up to 10% higher than PCN
CAP 698 SEP/MEP data
- Take off field Length limitations
If no clearway or stopway:
TODR x 1.25 <= TORA
With clearway or stopway:
TODR <= TORA; and
TODR x 1.3 <= ASDA
TODR x 1.15 <= TODA
CAP 698 SEP/MEP data
- Take-off safety factors
Grass up to 20cm - dry
Grass up to 20cm - wet
Paved - wet
1% upslope
Grass up to 20cm - dry: x1.2
Grass up to 20cm - wet: x1.3
Paved - wet: x 1.0
1% upslope: +5%
CAP 698 SEP/MEP data
- Finding ceiling
In climb chart look for altitude of 300ft/min climb ability.
This is the altitude limit you can consider for calculations around ability to glide to safety in case of engine failure.
CAP 698 SEP/MEP data
- Landing safety factors
LDA restriction
<20cm grass, firm soil
Wet
1% downslope
LDA restriction: Use 70% LDA
<20cm grass, firm soil: x1.15
Wet: x1.15
1% downslope: +5%
CAP 698 SEP/MEP data
- Obstacle clearance height if nothing given
50ft
[Unless question asks for ground roll!]
CAP 698
- Using MRJT1 brake cooling schedule for RTO
Ignore the brake configuration section, go straight through reference line (i.e. no reverse thrust credit etc.)
Stall speed definition terminology
“1-G stall speed”
“Where aeroplane can develop lift equal to weight” or “Load factor of 1”
“Where CLmax is reached”
Best endurance altitude for:
- Piston
- Turbo-prop
- Jet
Piston: Low (sea level)
Turbo-prop: Medium
Jet: High
