LESSON 14 Chapter 13 Landing Performance ANA Chapter 2

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Flight Theory

Chapter 13

Landing Performance

Dole discusses 4 important landing factors

  1. Approach paths and approach speeds
  2. Hazards of hydroplaning
  3. Deceleration during landing
  4. 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.

http://www.youtube.com/watch?v=RnYcPJTtV1A&feature=player_embedded

Braking action factors are:

  1. Tire material
  2. Tread design and wear
  3. Runway surface material and condition
  4. Amount of braking applied (wheel slippage)
  5. 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

  1. Dynamic
  2. Viscous
  3. 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

 

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