Joined: 11 Dec 2005
|Posted: Mon Apr 09, 2012 12:51 pm Post subject: Descent and Landing Performance
| Glide Descent:
Due to similar forces, descent is like a negative climb.
Climb angle was Sin gamma = T-D/W
So Descent Angle becomes:
Sin gamma = D-T/W
Normal descent is flown as a glide (idle throttles).
Since thrust is zero in a glide the formula for glide angle becomes:
Sin gamma = D / Weight
Lift can be taken equal to weight (though it is actually slightly less), the foumula becomes:
Sin gamma = Drag / Lift
Using the co-efficients of drag and lift.
Sin gamma = CD/CL
Thus glide angle is most shallow (best glide angle) where CD/CL is a minimum. The best glide angle occurs at VMD this will give the best range in a glide.
Glide angle itself is independent of aircraft weight as CD/CL is unchanged with weight, provided the aircraft is glides at VMD.
However, a heavier aircraft will have a higher value of VMD.
Thus it will be flying faster down the same glide path.
Rate of descent will be higher at the higher weight.
At lower weights VMD will be lower - flying the correct VMD speed - the angle of glide will be the same - rate of descent will be less.
Glide Angle Calculation:
Q. An aircraft with a lift/drag ratio of 20:1 is gliding at VMD. What ground distance, in nautical miles, will be covered in 5000ft of descent in still air conditions?
Sin gamma (Glide Angle) = CD/CL
i.e. for every 1 feet descent you cover 20 feet on the ground.
for 5000ft descent the aircraft covers 20 x 5000
= 100,000 ft
= 100,000 / 6080
= 16.45 nm
Factors Affecting the Glide Angle:
Glide angle = CD / CL
Increase in Lift = Reduction in glide angle.
Increase in Drag = Increase in glide angle.
Flaps increase Lift + Drag so CD/CL does not improve.
A Speed faster or slower than VMD will lead to steeper glide angles.
Increase in mass will increase VMD. As long as VMD is flown, glide angle remains the same.
Headwind will reduce gliding range. Increasing airspeed will give the best overall result.
In a Tailwind, a reduced speed will give the best result.
During driftdown the operating engine is at the max continous thrust.
When thrust is involved then it is not the glide angle (thus not independant from weight).
It becomes the Descecnt Angle:
= Drag - Thrust / Weight
As the aircraft descends, the thrust increases and the driftdown angle becomes progressively more shallow until it reaches zero and the aircraft stabilises.
The effect of weight is to make the driftdown angle steeper at heavier weights and to make the stabilising altitude lower.
Driftdown is flown at Vx.
For jets Vx is VMD. It is the best (least) angle of descent speed.
For props Vx is about 1.1Vs
Rate of Descent:
It is the opposite of rate of climb.
Rate of Climb = (Power Available - Power Required) / Weight
Rate of Descent = (Power Required - Power Available) / Weight
It increase as speed increases and as drag increases.
Gliding for Endurance:
Minimum power required is found at VMP.
Glide for Endurance = Fly at VMP
"Missed Approach" is defined as a go-around from at or above DH with one engine inoperative.
This is also called the "Approach Climb".
Aircraft configuration for the Class A aircraft will be:
• Go-around thrust on remaining engines
• Landing gear retracted
• Approach flap set
"Baulked" or "Balked" landing is a go-around from below DH, possibly in the flare, with all engines available.
This is also called the "Landing Climb".
The aircraft configuration for the Class A aircraft will be:
• Go-around thrust all engines
• Landing gear down
• Landing flap set
Minimum control speeds with one or two engines inoperative are called VMCL1 or VMCL2.
VMCL is the minimum speed at which it is possible to maintain control of the aeroplane within defined limits. whilst applying variations in power.
At VMCL (following loss of a critical engine) it should be possible to maintain straight flight with an angle of bank of not more than 5 deg towards the live engine(s) and there must (in addition) be sufficient lateral control to roll the aeroplane away from the failed engine(s) through an angle of 20 deg in less than 5 seconds when starting from a steady straight flight condition.
VMCL is be established with:
• Aeroplane in the most critical configuration (or each configuration) for approach and landing with all engines operating.
• Most unfavourable centre of gravity.
• Aeroplane trimmed for approach with all engines operating.
• Most unfavourable weight or as a function of weight.
• Propeller of the inoperative engine in the position it achieves without pilot action, assuming the engine fails while at the power/thrust necessary to maintain a 3 degree approach path angle.
• Go-around power/thrust setting on the operating engine(s).
VMCL will be determined at minimum landing weight.
Landing distance required is split into the Landing Airborne Distance and the Landing Ground Run.
Landing begins from the point where the aircraft crosses the threshold at a screen height of 50 feet at VREF to the point where it comes to rest.
In steep approaches the screen height may be reduced to 35ft.
Failure of Anti-Skid system can increase the Landing Distance Required by 50%
Failure of Reverse Thrust System can increase the Landing Distance Required by 10%
Reverse thrust is not considered on a dry runway but is considered on a wet or contaminated runway.
CS25 does not consider the use of reverse thrust when schedulinf the Landing Distance Required.
Factors Affecting Landing Ground Run:
For stopping the aircraft kinetic energy must be dissipated.
Kinetic energy = 1/2 M V^2
M = mass
V = speed
Thus Mass and Speed are the basic two things that affect the landing distance.
Kinetic energy = V squared
That means a rise in threshold speed, VREF or VAT means a significant increase in kinetic energy and a consequent increase in LDR.
Speed at touchdown is a true groundspeed and is affected in turn by air density, wind and aircraft weight.
Maximum increase in VREF (when applying wind corrections) is generally limited to 15-20 kts.
Above this capability of brakes will be exceeded.
Touchdown speed results from the threshold speed, VREF.
This is based on a function of the stall speed of the aircraft.
1.23 VSRO for Class A
1.3 VSO for Class B
And it should be above VMCL.
So as mass increases - threshold speed increases.
Thus increase in mass = Increases in mass + Speed in the kinetic energy formula = Increases in the landing distance.
Low air density - High TAS for a given IAS - High Ground Speed - Increase in Landing Distance.
Headwind = Lower Ground Speed = Decrease in Landing Distance.
Taiwind = Higher Ground Speed = Increase in Landing Distance.
For increasing the safety margin (while calculating) use:
- Half of the headwind component.
- One and a half times the tailwind component.
Brake effectiveness depends on the friction.
Aquaplaning / Hydroplaning will increase the LDR
EU OPS requires an additional 15% factor to be imposed on the landing distance required when the runway is wet or contaminated unless Flight Manual information allows a reduction below this.
Down sloping runway will increase and up sloping runway will decrease LDR.
CS require slopes of less than 2% to be ignored in landing performance calculations.
Landing Safety Factors:
Class A jet aircraft must land and stop within 60% of the Landing Distance Available (LDA).
Class B and Turboprop aircraft must land and stop within 70% of the Landing Distance Available (LDA).
There is no distinction in safety factors between destination and alternate aerodromes.
As factors these are:
Gross Dry LDR = LDA / 1.67
Gross Wet LDR = LDA / 1.92 (1.67 x 1.15)
Gross Dry LDR = LDA / 1.43
Gross Wet LDR = LDA / 1.64 (1.43 x 1.15)
A scheduled landing weight is a weight worked out well in advance of the flight to determine how much payload can be put on board and is distinct from a landing weight or distance planned once airborne.
Scheduled landings do not allow temperature to be taken into account when determining the Field Length Limit but require it to be used for the climb limit (WAT).
This is interpreted as a requirement to use ISA deviation zero for scheduled or planned landings.
EU OPS also requires that the most limiting of still air and forecast wind conditions be considered at both destination and alternate aerodromes.
For a single runway with no slope the most limiting case is always the still air case.
In Class A, slopes of less than 2% are ignored.
So the maximum scheduled landing weight on a single runway would be worked out with:
- Still air.
- ISA deviation zero.
- Ignoring the slope.
A Class A aircraft is to make a scheduled landing at an airport which has two runways, A and B. The maximum landing weights have been calculated to find the following information:
FLL weight runway A still air = 174,000 kg
FLL weight runway B still air = 162,000 kg
FLL weight runway A with wind = 186,000 kg
FLL weight runway B with wind = 177,000 kg
WAT limited landing weight = 192,000 kg
Determine the maximum scheduled landing weight?
The best (greatest) landing weight in still air is 174,000 kg
The best (greatest) landing weight in forecast wind is 186,000 kg
The more restrictive of the two is 174,000 kg.
The WAT limit is greater at 192,000kg
So the Field Length Limit of 174,000 kg is the max scheduled landing weight.
Scheduling a landing is based on the following regulation about "Landing Field Length Limits":
a) An operator shall ensure that the landing mass of the aeroplane for the estimated time of landing at the destination aerodrome and at any alternate aerodrome allows a full stop landing from 50 ft above the threshold.
c) When showing compliance with sub-paragraph (a) above, it must be assumed that:
1) The aeroplane will land on the most favourable runway, in still air
2) The aeroplane will land on the runway most likely to be assigned considering the probable wind speed and direction and the ground handling characteristics of the aeroplane. and considering other conditions such as landing aids and terrain. The landing weight must be planned so the aircraft can land in both still air and forecast wind conditions.
The most favourable runway will be a longest one (allowing maximum payload).
Therefore to solve the question, we start of by selecting the higher weight (which relates to a longer runway).
It is not about scheduling a landing in a way that all runways can be used for landing, if need be.