W029n.1

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Stopping on Slippery Runways
Paul Giesman Principal Engineer Flight Operations
Engineering Boeing Commercial Airplanes
2
Landing on Wet/Slippery Runways

• Landing
• Information - Condition Reporting
• Approach, Flare, and Touchdown
• Stopping
• Recommended Landing Procedure

3
Information

• Weather - winds, gust - approach speed
• Runway condition is typically provided three ways
• PIREPs (pilot reports) - braking action - good,
  fair, medium, poor, nil
• Description of runway condition
• Snow, wet, slush, standing water, sand treated
  compact snow etc.
• Reported friction based on Ground Friction
  Vehicle Report
• 30 or 0.30 etc.

4
Evaluation of information

• Flight crew needs to evaluate the information
  available to them
• Time of report, possible changing conditions
• Information may be conflicting
• For example
• Braking action is good, runway description is
  slush covered
• Measured friction is 0.35, runway description is
  slush covered
• If runway is reported to have slush/standing
  water covering, the flight crew should be
  suspicious of braking action reports and measured
  friction

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Slush/Standing Water Report

• Hydroplaning(aquaplaning) is possible
• Ground friction measuring vehicles are unreliable
  when the runway is covered with a depth of
  contaminant that exceeds
• Water - 1 mm
• Slush/wet snow - 3 mm
• Snow - 2.5 cm

Reference - FAA AC 150.5200-30A
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Landing Performance Data Available to Crews
(Boeing OM - Section PI)

• Boeing performance data is provided for pilot
  decision making
• Information published as a function of Reported
  Braking Action
• Good - Wet runway, JAR defined compact snow
• Medium - Ice, not melting
• Poor - Wet melting ice
• For landing, Boeing recommends the use of the
  data labeled poor for slush/standing water due
  to the possibility of hydroplaning

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Sample OMPI Slippery Runway Landing Data
Data provided for different braking actions and
configurations
Reported Braking Action Dry Good Medium
Poor
Braking Configuration Max Manual
Braking Autobrake Setting 2 Autobrake Setting
3 Max Autobrake Setting

• Autobrakes are recommended on a slippery runway.
  
• Medium, 3 or 4 are recommended depending on
  airplane

Sample data is from the 737 OM
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Sample OMPI Slippery Runway Landing Data

• Adjustments for
• Weight
• Altitude
• Wind
• Approach speed
• Slope
• Reverse thrust

Sample data is from the 737 OM
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Sample OMPI Slippery Runway Landing Data
Actual (unfactored) distances are shown Based on
flaps 40, VREF40 approach speed Landing distance
required includes 1000 ft of air distance 1200
for 747 Includes 2 engine reverse thrust
Note JAROPS data includes a factor of 1.15
Sample data is from the 737 OM
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Crosswind Guidelines

• Published in the Flight Crew Training Manual
• Guidelines, not limitations

Reference Boeing Flight Crew Training Manual -
747-400
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Landing on Wet/Slippery Runways

• Landing
• Information
• Approach, flare, and touchdown
• Stopping
• Recommended landing procedure

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Landing Approach, Flare, and Touchdown

• Objective
• Position the airplane on the runway at the target
  point at the minimum speed for the existing
  conditions.
• Minimize the air distance
• Maximize the stopping distance available
• Factors that influence air distance
• Flare technique
• Approach speed
• Approach path

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Landing Approach, Flare, and Touchdown

• Land in touchdown zone
• Do not allow the airplane to float. Fly the
  airplane onto the runway and accomplish the
  stopping procedure.
• Do not attempt achieve a perfectly smooth
  touchdown. Do not hold the nose wheel off the
  runway after touchdown.
• After main gear touchdown, begin to smoothly fly
  the nose wheel onto the runway by relaxing aft
  control column pressure.

Reference Boeing Flight Crew Training Manual
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Deceleration Rate ComparisonAir Versus Ground
Based on 747 at operational landing weight Wheel
brakes, spoilers, and reverse thrust as noted
10.0
Dry
Dry
8.0
Dry
Wet
Wet
6.0
Decel Kt/sec
Wet
Icy
Icy
4.0
Icy
2.0
1/2"
0.0
4-Engine Reverse Thrust
Floating or Aerobraking
No Reverse
2-Engine Reverse Thrust
Note On airplanes with more effective reversers
the ratio of ground attitude deceleration can be
9-10 times more than floating deceleration.
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Effect of Floating
Normal VTD 10
20
Normal VTD
15
Excess approach speed - knots
10
Normal VTD - 10
5
0
0
1,000
2,000
3,000
4,000
5,000
6,000
Increase in air distance - feet

• Excess speed
• Bleeding off excess speed during flare will
  increase air distance by
• 150 to 200 feet / knot of speed reduction
• 225 to 275 feet / second of additional air time

Based on 747 at operational landing weight - 4
thrust reverser
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Excess Speed at Touchdown Effects Stopping
Distance

• Greater touchdown velocity causes longer ground
  distance

Same weight, same runway conditions
VTD
Stop
VTD 10
Stop
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Landing Distance Increase due to Excess Touchdown
Speed
Medium
20
Good, wet
Dry
15
Excess touchdown speed, kt
Poor, wet ice / hydroplane
10
5
0
0
200
400
600
800
1,000
1,200
Increase in stopping distance, ft
Based on 747 at operational landing weight -
4-engine reverse thrust
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Excess Threshold Height
Excess Height
Increased in Distance to Touchdown
Normal 50 ft
80
60
Based on a 3-degree glideslope
Excess height at threshold, ft
40
20
0
0
500
1,000
1,500
Increased in distance to touchdown, ft
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Landing - Stopping / Roll out

• Objective
• Stop the airplane within the remaining runway
  available.
• Factors affecting stopping distance
• Reduced runway friction capability
• Wet
• Standing Water / Slush
• Ice / compact snow
• Effectiveness of stopping devices
• Thrust reversers, ground spoilers, wheel brakes

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Runway Friction Capability

• Hydroplaning
• Viscous - normal wet runway friction
• Dynamic - planing of the tire on standing
  water and slush
• Reverted rubber - locked wheel hydroplaning

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Viscous Hydroplaning Normal Wet Runway Friction
Thin film of water acts like a lubricant. The
microtexture (sandpaper type roughness) of the
runway surface breaks up the water film and
greatly improves traction.
Dry runway
Airplane braking coefficient
Rougher microtexture
Smoother microtexture
Ground speed, kt
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Dynamic Hydroplaning Commonly Called
Hydroplaning, Aquaplaning
At high speeds the tire planes on deep
slush/standing water. Tire grooves and
macrotexture (stony or grooved surface) help
drain water from the footprint and improve
traction.
Dry runway
Airplane braking coefficient
Tire pressure, psi
VHP
8.63
Nil braking above 90 of dynamic hydroplaning
speed
.9VHP
VHP
Ground speed, kt
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Reverted Rubber Hydroplaning Locked Wheel
When a tire locks up on a smooth wet or ice
surface, the friction heat generates steam. The
steam pressure then lifts the tire off the
runway, and the steam heat reverts the rubber to
a black gummy deposit.
Steam
Steam
Reverted rubber hydroplaning is not an issue on
post-1980 airplane designs due the improvement in
anti-skid system hydroplane protection.
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Effect of Runway Condition on Stopping Distance
160
Medium
140
Dry
Wet, good
120
100
Poor, wet ice / hydroplane
Ground speed, kt
80
60
40
20
0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
Distance to stop, ft
Based on 747 at operational landing weight -
Wheel brakes, spoilers, 4-engine reverse thrust
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Effectiveness of Stopping Devices

• Dry runway - Wheel brakes are the most effective
  stopping devices
• Lift reduction due to spoiler deployment
  contributes greatly to the generation of
  effective stopping force due to wheel brakes

Effect of ground spoilers
Reverse thrust
Aero drag
Wheel brakes
Ground spoilers
49000
54000
168000
No ground spoilers
49000
35000
106000
0
100,000
200,000
300,000
0
Stopping force, lb
Based on 747 at operational landing weight -
Wheel brakes, spoilers, 4-engine reverse thrust,
120 knots
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Effect of Spoilers
Total stopping force, lb Reverse thrust aero
drag wheel brake
Spoilers Spoilers Spoiler effect deployed
stowed stopping force Dry 270,100 190,400 29
Good 194,400 142,300 27 Medium 148,500 113,200
24 Poor 125,600 98,600 21
Based on 747 at operational landing weight -
Wheel brakes, spoilers, 4-engine reverse thrust -
120 knots
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Effectiveness of Stopping Devices

• Slippery runway - thrust reverser and aerodynamic
  drag become dominate stopping force as runway
  slipperiness increases

Percentage of stopping force due to drag and
reverse thrust
Reverse thrust
Aero drag
Wheel brakes
Poor - wet ice/ hydroplaning
80
70
Medium
55
Good, wet
35
Dry
0
100,000
200,000
300,000
Stopping force, lb
Based on 747 at operational landing weight -
Wheel brakes, spoilers, 4-engine reverse thrust -
120 knots
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Automatic Braking

• Boeing recommends autobrake when landing on a
  slippery runway
• Setting 3 or 4 for wet or slippery runway
• Actual setting dependent on model
• Autobrake assures prompt application of the
  brakes after touchdown
• Autobrake performance capability is limited by
  the runway friction capability

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AFM Landing distance
VRef
Stop
Demonstrated dry capability
VRef
1.67 factor
Stop
FAR dry
VRef
1.15 wet factor
1.67 factor
Stop
FAR wet
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Distance Comparison Book Data
100
VRef
1.15 wet
1.67 factor
AFM - FAR wet
Stop
Stop
70
OM - Max manual
Good
Stop
90
OM - A/B 3
Stop
Medium
OM - A/B 3
90
OM - A/B 3
Stop
Poor
110
OM - Max manual
Based on 747 at operational landing weight - OM
data based on unfactored, 4-engine reverse
thrust assuming a 1,200-ft touchdown point.
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Distance Comparison50-ft High and Extended Flare
(Floating)
100
VRef
1.15 wet
1.67 factor
AFM - FAR wet
Stop
Vref 10
Stop
VTD normal
1200 ft
Max Manual
100
Good
Vref 10
Stop
VTD normal
1200 ft
120
A/B 3
Vref 10
Stop
VTD normal
1200 ft
A/B 3
120
Medium
Vref 10
A/B 3
Stop
VTD normal
Poor
1200 ft
140
Max Manual
Based on 747 at operational landing weight -
4-engine reverse thrust - extended flare data is
based on a 10-kt high approach speed bled off
during the flare maneuver.
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Distance Comparison50-ft High and Extended Flare
(Floating) Plus No Reverse Thrust
100
VRef
1.15 wet
1.67 factor
Stop
AFM - FAR wet
Vref 10
Stop
VTD normal
1200 ft
Max Manual
110
Vref 10
Good
Stop
VTD normal
1200 ft
130
A/B 3
Vref 10
Medium A/B 3
Stop
VTD normal
1200 ft
140
Vref 10
Poor A/B 3
Stop
VTD normal
1200 ft
180
Max Manual
Based on 747 at operational landing weight -
4-engine reverse thrust - extended flare data is
based on a 10-kt high approach speed bled off
during the flare maneuver.
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Summary Recommended Procedures

• Information
• Evaluate all the information before the approach
• Wind, weather, runway condition, etc.
• If runway conditions warrant, review the
  performance data to ensure the runway length
  exceeds the expected stopping distance by an
  adequate margin

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Summary Recommended Procedures (continued)

• Prepare to land the aircraft
• In the touchdown zone
• 1,000-ft target
• On centerline
• With minimal lateral drift
• Without excess speed
• Normal speed additives
• Arm auto spoilers and auto brakes as appropriate
• Assures prompt stopping effort after touchdown

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Summary Recommended Procedures (continued)

• Flare and Touchdown
• Flare should lead to a firm touchdown
• Extended flare will extend touchdown and delay
  braking
• Lower the nose as soon as main gear touches down
• Increases load on the gear

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Summary Recommended Procedures (continued)

• Raise spoilers as soon as possible after
  touchdown (confirm auto spoiler deployment)
• Increase load on the gear
• Initiate braking once spoilers have been raised
  and nose wheels have contacted the runway
• Apply brakes smooth and symmetrically

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Summary Recommended Procedures (continued)

• Initiate reverse thrust as soon as possible after
  touchdown
• Target the rollout to stop well short of the end
  of the runway
• Leave margin for unexpectedly low friction due to
  wet rubber deposits or hydroplaning