LESSON 14 Chapter 13 Landing Performance ANA Chapter 2
Chapter 13
Landing Performance
Dole discusses 4 important landing factors
- Approach paths and approach speeds
- Hazards of hydroplaning
- Deceleration during landing
- The distance required to stop the aircraft
Pre-landing Performance
The glide is where an airplane will find equilibrium when the thrust force is reduced to 0.
So in an engine out glide we need several key pieces of information:
How far can we glide?
How long can we remain airborne?
What will the sink rate be?
Can the plane glide to the selected spot?
Can a successful landing be made?
Forces in the glide
Both lift and drag act as they normally do through the vertical and longitudinal axis.
Weight however also acts directly toward the center of the earth.
Using vector analysis we can form a right 90 triangle below the aircraft.
Forces in the glide
The glide path is called gamma γ
The component of the weight that acts in the direction of the vertical axis is
Remember it acts through the center of gravity and opposes lift.
The component that acts along the longitudinal axis is
This component acts through the cg and opposes drag
Forces in the glide
Force equations for steady state glide are:
The flight path angle must be at a minimum to achieve best glide.
This only happens at L/Dmax. If the pilot tries to stretch the glide by pulling up the nose the glide range will decrease.
Forces in the glide
L/Dmax remember is achieved as a function of weight
So a lighter plane must be flown at a lower speed to achieve the L/Dmax α
If flown at L/Dmax, both the heavy and the lighter aircraft will glide to the same spot
The glide ratio vector diagram supports this notion
The TAN of γ is opposite side/adjacent side where:
Drag or vertical V is the opposite side
Lift or horizontal V is the adjacent side
The glide angle is found by dividing drag by lift
Or by taking vertical V divided by horizontal V
The Landing Approach
Remember that the approach speed is usually not the absolute minimum speed.
It may have factors such as weight, stall speed, minimum controllable speed, TA vs TR or PA vs PR added in
Most GA airplanes have a factor of 1.3Vso
That equates to 130% of the stall speed in the landing configuration
On the approach the prop plane will have an advantage in that thrust is readily available almost immediately and induced airflow will provide some lift
The Landing Approach
On the jet plane, it may take some time for the engine to spool up and generate thrust
There is no induce airflow for most jets with the exception of a few like the C-17.
The Landing Approach
One thing you do have mainly in jets is the vertical component of thrust on the approach.
Because jets are flown at higher AOA the component of thrust is usually higher than that for prop powered aircraft.
Variables of airspeed, glide slope and drift must be accounted for to enable a stable final approach
The stable final approach allows for a better and safer landing every time
High bank angles should be avoided due to higher stall speed in the turn
Also redistributing the lift requires large changes in α which in turn change drag values which require changes in thrust values
This will reduce the precision the pilot is able to maintain on final
The Landing Approach
Vertical approach paths must be taken into account as well
Too high of an approach requires reduction in thrust and decrease in pitch which destabilizes the approach
Too low of an approach requires an increase in thrust and an increase in pitch which destabilizes the approach
In these circumstances, the pilot has 3 options:
Make the necessary pitch and power changes
This may result in high sink rates or high power approaches
Continue the normal approach and land long or short
Execute a go around and try again, hopefully not making the same mistakes on the previous approach
The Landing Approach
The long shallow approach requires high power and high α
Because of instant power and induced flow from the prop, you may be able to get away with this technique
However with jets, only the vertical component of thrust is of benefit
The high drag offsets the benefits of the vertical component of thrust
In either case, an engine failure on approach (think checkride) could result in disaster
After engine failure, high sink rates, being force to lower the pitch, and stall before getting in the flare are all possibilities here
Landing Distance
You can think of the landing distance in terms of kinetic energy that must be dissipated before the airplane is stopped.
Remember, mass is in slugs and velocity is in fps:
You can dissipate energy by using the brakes, aerodynamic braking or by transferring the energy into the ground
In fact, if you transfer enough energy into the ground, you can shear off the gear and really shorten your landing distance
So after your next less than perfect touch down just tell your instructor you’re dissipating the KE for a shorter roll out
In an emergency landing, shearing off wings and/or gear may dissipate the KE and reduce the g forces to a survivable level
Which is why you almost always want to land gear down
Landing Distance
Landing distance is directly affected by weight
Double the weight and double the landing distance
While the effects of velocity are more pronounced
Double the landing speed and quadruple the energy to be dissipated
A 10% increase in landing speed will result in at least a 21% greater landing distance
Forces on Aircraft during landing
Heavy aircraft do not use much aerodynamic braking.
The large mass of these airplanes need a more effective way to stop in a short amount of space.
Therefor thrust reversers and anti skid brakes are the choice of these behemoths. Yes that’s right I said behemoths.
Forces On Aircraft During Landing
The diagram makes 3 assumptions:
Aerodynamic braking is used
Brakes are not used until the nose wheel touches
No lift is generated in the 3 point attitude
Forces on Aircraft during landing
Smaller planes and non thrust reverser equipped jets can take advantage of aerodynamic braking.
Because drag varies at the square, at half your landing speed the drag is about one quarter what it was at touchdown.
The general rule of thumb is to use aerodynamic braking for about one quarter of the landing roll out then use wheel brakes for the rest.
Forces on Aircraft during landing
Once the airplane is on the ground, hold the yoke full back to transfer as much weight as possible to the mains to give maximum normal force to increase brake effectiveness.
Braking action factors are:
- Tire material
- Tread design and wear
- Runway surface material and condition
- Amount of braking applied (wheel slippage)
- Amount of normal force (squeezing force between tires and runway)
Braking action factors
Vary any one of the previously mentioned 5 things and landing rollout could be dramatically affected.
Landing surface
One should consider the surface before landing.
Application of brakes may not be possible such as ice or snow covered runways.
If you had a wheel inadvertently locked (dumbshit) and a bare spot popped up you could blow a tire and cause a side swerve followed by runway exodus.
Braking action
In this case aerodynamic braking should be used to the fullest extent and plan on having a longer runway roll than usual.
The amount of normal force is critical to brake effectiveness. The more normal force, the better your stopping power.
Braking Action
Using an average value of .5 for Coefficient or braking friction produces about 16 fps deceleration
Values for Coefficient of braking friction vary from a max of .8 to .2 or .1 on ice
Aerodynamic braking produces a deceleration of about 8 fps
Equations
The equations for landing are the same as for takeoff.
Weight, altitude, and wind all have the same effects.
Hydroplaning
This occurs when there is a build up of water (or rubber) between the tire and the surface.
There are 3 types of hydroplaning
- Dynamic
- Viscous
- Reverted rubber
Dynamic hydroplaning
This is when a wedge of water has separated the wheel from the runway.
When a rolling tire is analyzed, the ground friction causes a spin up moment resulting in tire rotation.
This moment causes the vertical ground reaction line to shift forward of the axle.
Dynamic hydroplaning
A wedge of water builds under the tire until the tire is lifted completely off the runway.
The equation for that speed is:
Where:
VH = velocity of hydroplaning
9 = constant
P = tire pressure
This type of hydroplaning usually only occurs in really heavy downpours.
Smooth tires and smooth runway surface will induce hydroplaning at lower water depths.
Viscous Hydroplaning
This variety is much more common than dynamic and occurs at lower speeds and lower water depths.
This is where a thin film of water lubricates the runway and contact with the pavement is partially lost.
This may occur on 32R where there is a lot of rubber deposits and no place for the water to go.
Reverted Rubber Hydroplaning
When the pilot locks up the brakes in such a manner as to cause the friction to heat the tire to the point of melting, the tire is said to have reverted to its natural state. (dumbshit in a state of panic)
Runway Surface
Wind
Since approximately the same approach speed is used no matter what the wind, ground V is the main concern here
A headwind 10% of landing speed will reduce the landing distance 19%
A tailwind 10% of landing speed will increase the landing distance 21%
Runway Slope
The component of weight acting along the inclined path is identical to the takeoff discussed earlier
However, the magnitude is not as great
Therefor it is better to land downslope with a headwind than upslope with a tailwind
Altitude
The landing is not as greatly affected by altitude as the takeoff. Engine performance is not such a factor
A ground roll equation generic is .3V2 where V is landing velocity in true
Landing distance increases 3% for each 1000’ of altitude above the sea level value
Remember that you will have the same IAS but higher TAS and Ground speed
Lake County Airport at Leadville CO is the highest public use airport in the USA at 9934 feet
Runway is 6400 feet long
Temperature
The biggest thing to remember is the density altitude
Furnace Creek Airport is the lowest airport in the USA at -210 feet
Runway length is 3065 feet
Highest ever recorded temperature was taken at Furnace Creek 134 F
Airplane Weight
Weight is one of the principle items that will effect landing distance
The higher the weight, the longer the distance obviously
The higher weight will increase the stall speed and decrease the stall margin on approach
However, it will also allow better braking effectiveness because you have more weight on the wheels on touch down.
A 10% increase in weight requires:
5% increase in approach speed
10% greater landing distance
21% greater amount of KE to be dissipated
A Lesson in Weight and Balance, from my inlaws