Boeing Slippery Runways Notes

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159TH International Air Safety Seminar25 October 2006 – Paris, France

Airplane Deceleration on Slippery Runways

What You Should Know

Mark H. SmithMark H. SmithBoeing Air Safety Investigation

• During the investigation into the 737 landing overrun accident at Chicago’s Midway Airport in December 2006, it was discovered that there is misunderstanding and confusion among some operators about several issues relating to airplane performance on slippery runways. This presentation is about the airplane and its associated performance data. Specifics of the accident will not be discussed here because this is still an open NTSB investigation. I hope this information will be useful to you and your operations.

• For simplicity and continuity this presentation will use the Boeing 737-700 airplane as the specific example.

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Airplane Deceleration On Slippery RunwaysAgenda

Certified vs. Advisory Landing Distance DataCertified vs. Advisory Landing Distance Data

Airplane Braking Airplane Braking vsvs. Runway Friction. Runway Friction

Autobrake Autobrake vsvs. Manual Brakes. Manual Brakes

• Three specific topic areas will be discussed:• Certified vs. Advisory Landing Distance Data• Airplane Braking vs. Runway Friction• Autobrake vs. Manual Brakes

• First, I will discuss the difference between the two different sets of Landing Distance Data that are provided by Boeing.

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“En route”“Unfactored”“Operational”

“Dispatch”“Factored”

Also Called:Certified DataCertified Data Advisory DataAdvisory Data

Landing Distance DataData Provided by Boeing

Two sets of landing data are provided.

Reqmt’s: Parts 25 & 121 Part 121

Location: Airplane Flight Manual (AFM)

Quick Reference Handbook (QRH)

• Boeing publishes two different sets of landing data for operators, referred to here as the “Certified Data Set” and the “Advisory Data Set.” Other terms are often used to refer to these same two data sets.

• The Certified Data has also been called “dispatch” or “factored” data, and is published in the Airplane Flight Manual or AFM.

• The Advisory Data has also been called “en route.” “unfactored” or “operational” data. The Advisory Data is located in the Quick Reference Handbook or QRH, which is located with the flight crew in the flight deck.

• For consistency throughout this presentation, I will use the terms “Certified Data” and “Advisory Data.” A blue and green color coding is used to help differentiate between the two data sets.

• The certified data set is required for type certification under Part 25 of the regulations. Both sets of data are required under Part 121 of the regulations.

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Certified DataCertified Data Advisory DataAdvisory Data

Landing Distance DataData Provided by Boeing

The two data sets are for different purposes and uses.

Use:

Purpose: Provide factored landing distance as required by regulators

Determine landing distance requirements prior to dispatch

Provide actual landing distance capability of the airplane

Determine landing distance for operational decisions while en route

• Each set of data has its own distinct purpose and use. The Certified Data is factored data that is required by Part 25 of the regulations. This data is used to determine the landing distance requirements prior to dispatch.

• On the other side, the Advisory Data provides actual landing distance capability of the airplane for various runway conditions and airplane braking configurations. This data is used by the flight crew to determine landing distance capability for making operational decisions while en route.

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Landing Distance DataBaseline Demonstrated Distance

RunwayThreshold

Air Distance

TouchDown

TransitionDistance

Auto-Speedbrakes DeployedBrakes Applied

Stopping Distance

Stop

• I will now show how each of these data sets is derived. Understanding what is built into the data should help operators understand the benefits and limitations of each data set.

• At Boeing, the landing distance calculations are broken into three segments:

- the air distance is the distance from the runway threshold to touchdown.

- the transition distance is the distance allotted for deployment of automatic speedbrakes and initial brake application.

- the stopping distance is the distance required to come to a full stop.

• These three segment calculations are the basic building blocks for the data in both data sets.

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RunwayThreshold

Air Distance

TouchDown

TransitionDistance


Stopping Distance

Stop

Dry

50 ft

Distance from 50 ft 1sec Max. Manual BrakingBasis Distance

Basis for Certified Data Set

• Specific to the Certified Data set,

- The air distance is calculated from 50 ft above the threshold to the point of touchdown. This distance is different for each airplane model.

- Next, the transition distance is calculated for a 1 second time period.

- Lastly, the stopping distance calculations use maximum manual braking on a dry runway. Note that no reverse thrust is used.

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RunwayThreshold

Air Distance

TouchDown

TransitionDistance


Stopping Distance

Stop

Reverse

1000 feet 1sec Max. Manual BrakingBasis Distance

Dry

Basis for Advisory Data Set

• Specific to the Advisory Data set,

- The air distance is purposely set at 1,000 feet. This is an even number that is easily adjusted by operators, if desired, for their specific operations.

- The transition distance is calculated for a 1 second time period.

- The stopping distance is based on maximum manual braking on a dry runway with reverse thrust.

• The profile shown for the Advisory Data represents a “nominal”landing profile where auto-speedbrakes and reverse thrust are used as standard operating procedure for all landings.

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RunwayThreshold

Reverse

1000 feet 1sec Max. Manual BrakingBasis Distance

Dry

Basis for Advisory Data Set

Dry

50 ft

Distance from 50 ft 1sec Max. Manual BrakingBasis Distance

Basis for Certified Data Set

Air Distance

TouchDown

TransitionDistance


Stopping Distance

Stop

• Comparing the basis of the two data sets shown on this chart, note the following:

- The basis for both data sets is derived from Boeing flight test demonstrations

- Both data sets are based on a dry runway- Both data sets use maximum manual braking (rather than autobrakes)

• The most significant difference between the Basis Distances of the two data sets is the use of reverse thrust. By regulation, the Certified Data is not allowed to take credit for reverse thrust. The Advisory Data does use reverse thrust because this is the recommended standard operating procedure for landings. As can be seen, reverse thrust reduces the landing distance by 100-200 feet (<5%) on a dry runway with maximum manual braking.

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• Dry runway• Automatic Speedbrakes• Max manual braking• No reverse thrust

Stop

Basis Distance

FAR Dry

Basis Distance x 1. 67

CERTIFIED Data

Basis Distance

Basis Distance x 1. 67 x 1. 15

Basis Distance x 1. 67

Basis Distance

FAR Wet / SlipperyCERTIFIED Data

Landing Distance Data CERTIFIED Data Set

• For the Certified Data set, additional factors must be applied to the data before it is incorporated into the AFM. Shown at the top of the page is the basis distance that was developed on the previous charts for the Certified Data Set. The data has been re-scaled on this page to allow for application of factors.

• For the Certified Data set, the basis distance is multiplied by a factor of 1. 67 to get the regulatory distance for the FAR Dry landing distance.

• An additional factor of 1. 15 is applied to the FAR Dry distance to get the FAR Wet/Slippery landing distance.

• Only these factored distances are given in the AFM as required by regulation. These distances are factored only, and are not based on actual airplane performance. These are the distances used for dispatch of the airplane.

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Snow

Ice

Wet

Dry

• Dry runway• Automatic Speedbrakes• Max manual braking• With reverse thrust

ADVISORY DataDry Braking

Stop

1000 ft Reverse

Medium Braking

Poor Braking

Good BrakingBasis Distance

Landing Distance Data ADVISORY Data Set

• Shown above is the basis distance that was developed on the previous charts for the Advisory Data Set. The data has also been re-scaled to allow direct comparison to the Certified Data set. The basis distance is the distance used for DRY braking action in the QRH.

• The advisory data provided in the QRH also shows airplane capability for landing on a wet, snowy or ice-covered runway.

• Braking on a wet runway maps to Good braking action in the QRH. • Braking on a snowy runway approximately maps to Medium braking action in

the QRH. • Braking on an icy runway approximately maps to Poor braking

action in the QRH.

• In each of the conditions shown here, the same landing procedure is used with max manual braking and reverse thrust, and the only difference is the airplane braking action for the given runway surface. This data is used by the flight crew to determine landing distance capability for making operational decisions while en route.

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• Dry runway• Automatic Speedbrakes• Max manual braking• With reverse thrust

Good Braking

Medium Braking

Poor Braking

ADVISORY DataDry Braking

Stop

1000 ft Reverse

Snow

Ice

Wet

Dry

ReferenceDistance• Dry runway

• Automatic Speedbrakes• Max manual braking• No reverse thrust

Basis Distance x 1. 67 x 1. 15

FAR Wet / SlipperyFAR DryBasis Distance x 1. 67CERTIFIED Data

Stop

Basis Distance

Basis Distance

Landing Distance Data SummaryCERTIFIED Data vs. ADVISORY Data

• This slide shows a direct comparison of the two data sets. A Reference Distance line is included at the end of the FAR Wet/Slippery landing distance for comparison purposes. Note that the Wet/Slippery runway distance in the Certified Data falls between the MED and POOR braking performance in the Advisory Data.

• As a final reminder, the Certified Data is a factored data set and is not allowed to take credit for reverser thrust.

• Conversely, the Advisory Data set is airplane performance data that does use reverse thrust because it is the recommended standard landing procedure.

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Landing Distance Advisory Data

Quick Reference Handbook (QRH) contains airplane landing distance data for various runway conditions and brake configurations.

Good

Med

Poor

Dry

• Here is an example QRH page of the Advisor Data for the 737-700.

• Landing distances are given in this table for the various braking conditions discussed previously.

• Distances for braking using maximum manual braking or an autobrake setting are given for each braking condition.

• The landing distance for the reference weight is shown in the first column, with corrections shown in the other columns.

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Landing Distance Advisory Data

QRH landing distance data is based on the notes at the bottom of the page.

Relevant notes include:- two engine detent reverse thrust

- 1,000 ft of air distance

- actual (unfactored) distances (JAR Ops include 15% margin)

Based on these notes

• I would like to call your attention to the notes at the bottom of the page.

• The calculations for the chart use the assumptions listed in the notes at the bottom of the page. Relative to this presentation, these include:

- two engine detent reverse thrust- 1,000 ft of air distance- actual (unfactored) distances are shown

• Note that the example chart shown here is actual, unfactored distances. Charts provided for JAA Operators include an additional 15% for margin, as required by JAR Ops rules. Beginning October 2006 the FAA will also require the additional 15% margin for US operators. Flight crews should be aware of the assumptions built into the data set in use.

• Finally, these notes are always included on the Boeing QRH pages. However, these notes may not be readily apparent to flight crews that use laptop computers to output landing distance data, or for operators that re-format this information into their own QRH format.

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The second item on the agenda is a discussion of the difference between airplane braking action and runway friction measurement.

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Airplane Braking vs. Runway Friction Runway Condition Reporting

• Three methods can be used to report runway conditionAirplane Braking Action Report

Runway Description Runway Friction Measurement

• Each method uses a different descriptor (scale)

• There are three methods of reporting runway condition to landingflight crews:

•- Method 1 is the airplane braking action performance or report. - Method 2 is a verbal runway description. - Method 3 is the runway friction measurement.

• I would like to point out differences between these methods that crews should be aware of. The key message that we hope you will take from this section is that each of the methods uses a different descriptor or scale to describe the runway.

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Airplane Braking vs. Runway Friction Summary of Reporting Methods

Better Braking

Worse Braking

Dry

WetDry Snow

Compact Snow

Wet SnowSlush

IceWet Ice

Runway Friction Measurement

Runway Description

Airplane Braking Action Report

Good

Med

Poor

Dry

Good

Med

Poor

ICAO

• Here are the three runway condition reporting methods shown graphically for comparison purposes.

- Method 1 is airplane braking action, which is a pilot evaluation of the airplane braking action at the time of the landing, using the descriptor terms of Good, Medium or Poor.

- Method 2 is a verbal description of the runway surface, based onobservations by airport personnel.

- Method 3 is a quantitative measurement of runway surface friction, as measured by an airport ground vehicle.

• I would like to focus on the technical differences between Methods 1 and 3, to highlight how each is describing a different piece of the puzzle.

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Airplane Braking vs. Runway Friction Summary of Reporting Methods

Runway FrictionMeasurement

Runway Description

CalculatedAirplane Braking

Good

Med

Poor

Dry

Good

Med

Poor

ICAOBetter Braking

Worse Braking

Dry

WetDry Snow

Compact Snow

Wet SnowSlush

IceWet Ice

QRHData

0. 4

0. 3

0. 2

0. 1

0. 0

Calculated AirplaneBraking

Coefficient

μAirplane

1. 0

0. 8

0. 6

0. 4

0. 2

0. 0

MeasuredRunwayFriction

Coefficient

μRunway

• This slide shows the science behind Methods 1 and 3.

• For Method 1, Boeing assigns a scale to the descriptor terms used in the QRH in order to make the landing distance calculations. This scale is airplane braking coefficient, or Mu-Airplane, with a range from 0 to 0. 4. Mu-Airplane is a calculated value of how the airplane systems convert the available runway friction into effective stopping performance. This calculation takes into account airplane GW, lift, CG and antiskid system performance.

• Method 3 is the runway friction as measured by an airport groundvehicle. A scale called runway friction coefficient, or Mu-Runway, is used with a range from 0 to 1. This is the measured value of the point at which the tire will skid on the runway, and does not account for any airplane parameters (GW, CG, etc. ).

• Thus, Method 1 is calculated airplane capability and Method 3 is the measured tire skid point. Flight crews should be aware that these are two entirely different things and these scales cannot be directly compared.

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Deceleration Available MoreLess

Airplane Braking vs. Runway Friction Deceleration Available from Brakes

Slippery Runwayμ –Runway (Tire Skid Point)

initially, brakes will attempt full pressure but this would lock the wheels

μ -Airplane (Braking Capability)

Max Brakes

e.g., stand on the brake pedals

Worse

Better

Braking Conditions

μ -Runway (Tire Skid Point)

Dry Runway

• This slide graphically demonstrates the difference between Mu-Airplane and Mu-Runway. Deceleration Available is shown on the horizontal axis versus braking conditions shown on the vertical axis. Note that this graph shows deceleration rather than landing distance, and larger deceleration results in a shorter landing distance.

• For a dry runway, Mu-Runway is represented by the red bar shown at the top of the page. If the crew were to apply maximum brakes, the brakes would respond up to the maximum available pressure of thesystem. In this case, Mu-Airplane is represented by the associated orange bar.

• On a slippery runway, Mu-Runway is limited by the skid limit of the tire. If the crew were to again apply maximum brakes, the brakes would attempt to apply the maximum available pressure of the system. If this were allowed to happen though, the tires would lock up and skid on the runway.

continued on next slide

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Deceleration Available MoreLess


Max Brakes

Slippery Runwayμ –Runway (Tire Skid Point)

μ -Airplane (Braking Capability)

μ -Runway (Tire Skid Point)

Dry Runway


Worse

Better

Braking Conditions

Antiskidμ –Airplane (Braking Capability)

Method

Method

• Instead, the antiskid system reduces the brake pressure to ensure the tire does not skid. On a slippery runway, the difference between Mu-Runway and Mu-Airplane is primarily the antiskid system efficiency, which ranges from 80%-90%, depending on the conditions.

• On this slide, the end of the red bars represent the tire skid point for the conditions, which is what the ground friction vehicle is attempting to measure in Method 3.

• For the associated conditions, the ends of the orange bars represent the airplane braking capability, and this is what airplane performance is based on in the Boeing QRH in Method 1.

• Flight crews should be aware that Method 1 (Mu-Airplane) and Method 3 (Mu-Runway) are different things, and that no industry-accepted method is available to compare the two.

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Deceleration Availablefrom Brakes

MoreLess


Max Brakes


Dry

Antiskid

Good

Med

Poor

Braking Conditions

This shows the deceleration available from the brakes for the various braking actions given in the QRH. Note that the deceleration for Good is half of Dry, Medium is half of Good and Poor is half of Medium.

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MoreLess


DryGood

MedPoor

Decel. Available from Brakes

Deceleration Availablefrom Brakes

For reference in next section

I have now collected these together at the top of the page for reference in the next section.

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The final agenda item is a discussion on autobrake versus manual brake performance relative to reverse thrust use and runway braking action.

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Autobrakes vs. Manual BrakesAirplane Deceleration

Total airplane deceleration is affected by three factors:

- Aerodynamic drag - Wheel brakes - Reverse thrust

If used, the autobrake system- Targets a deceleration level- Brakes applied as required to reach target level

• During a landing, total airplane deceleration is affected by three factors:

- Aerodynamic drag - Wheel brakes - Reverse thrust

• In the previous section, only the portion of deceleration available from the wheel brakes was discussed. This section will review overall airplane deceleration during a landing, and will discuss how themethod of brake application (manual or autobrakes) affects airplane performance.

• With manual brakes, the brake system applies whatever brake pressure is commanded by the crew. If autobrakes are used, the autobrake system targets a predefined deceleration level and will apply the wheel brakes only as required to reach target level.

• This will be demonstrated graphically on the next few pages.

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Total Airplane Deceleration MoreLess


Braking AppliedMax Manual

Autobrake Max

Autobrake 2

BrakesDrag Reverse Thrust

BrakesDrag

• The graph above now shows Total Airplane Deceleration on the horizontal axis versus braking method on the vertical axis. Again, this graph shows deceleration rather than distance, and larger deceleration results in a shorter landing distance.

• For a landing using max manual brakes and no reverser thrust, the total deceleration would be the sum of the deceleration from drag and from brakes. The same landing except with reverse thrust would result in the sum of the deceleration from drag, brakes and reverse thrust. Thus, the deceleration from reverse thrust is additive when using manual brakes, resulting in a shorter landing distance.

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Autobrake Max

Autobrake 2


BrakesDrag


BrakesDrag


BrakesDrag

Deceleration levelvalid with or without

reverser thrust

Decel Target

• If autobrakes are used, the autobrake system will apply the wheel brakes until a predetermined target deceleration level is achieved.

• For a landing using Autobrakes Max and no reverser thrust, only enough braking is applied to meet the target deceleration. The same landing except with reverse thrust would not change the deceleration achieved. In this case, less braking would be applied by the autobrakes because reverse thrust is helping the airplane achieve the target deceleration.

• Using Autobrakes 2, a lower target deceleration is used, so lessbraking is needed than with Autobrakes Max. If using reverse thrust, very little braking would need to be applied to achieve the target deceleration.

• In this example, reverse thrust effects are not additive when using autobrakes. Therefore, the landing distance is unchanged whether or not reverse thrust is used.

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Autobrakes vs. Manual BrakesTotal Airplane Deceleration

Braking Applied

DryGood

MedPoor


Max Manual

Autobrake Max

Autobrake 2


Decel. Target

BrakesDrag


BrakesDrag


BrakesDrag

I now want to re-introduce the maximum deceleration available from the wheel brakes as a function of runway conditions. Overlaid at the top of the page is the maximum deceleration available from the brakes as was developed in part 2 of this presentation.

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DryGood

MedPoor



Autobrakes vs. Manual BrakesDry Braking


Autobrake Max

Autobrake 2


Decel. Target

BrakesDrag


BrakesDrag


BrakesDrag

The example shown on this plot is correct for landing on a DRY runway when full brake capability typically exists. Let us examine how this example changes if less deceleration is available from the brakes due to slippery runway conditions.

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DryGood

MedPoor



Autobrakes vs. Manual BrakesGood Braking


Autobrake Max

Autobrake 2


Decel. Target

BrakesDrag


BrakesDrag


BrakesDrag

• This is the same example, except with GOOD braking action available from the brakes.

• In the Max Manual braking case, less overall deceleration is available, but reverse thrust is still additive to the deceleration from drag and brakes.

• In the Autobrake Max case, the situation has changed. The autobrake system is no longer able to achieve the target deceleration because less brake capability is available. The autobrake system still attempts to meet the target deceleration and applies maximum available brakes. In this case, the deceleration from reverse thrust is additive when using autobrakes, resulting in a shorter landing distance.

• The Autobrake 2 case is unchanged, because the brake capability needed to meet the target decel is below the the max braking available, so the target decel can still be met. The landing distance for Autobrake 2 is valid with or without reverse thrust.

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DryGood

MedPoor



Autobrakes vs. Manual BrakesMedium Braking


Autobrake Max

Autobrake 2


Decel. Target

BrakesDrag


BrakesDrag


BrakesDrag

• This is the same example, except with MEDIUM braking action available from the brakes.

• As can be seen the deceleration from reverse thrust is additive when using autobrakes, resulting in a shorter landing distance.

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DryGood

MedPoor



Autobrakes vs. Manual BrakesPoor Braking


Autobrake Max

Autobrake 2


Decel. Target

BrakesDrag


BrakesDrag


BrakesDrag

• This is the same example, except with POOR braking action available from the brakes, and the landing distance for all braking cases is less if reverse thrust is used.

• As can be seen from these last few pages, reverse thrust becomes the most effective deceleration device as runway conditions deteriorate.

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Autobrakes vs. Manual BrakesSummary

Manual BrakesDry runway: Reversers are additive

Slippery runway: Reversers are additive

AutobrakesDry runway: Reversers not additive

Slippery runway: Reversers may be additive

Landing Distance Advisory Data in the QRH includes reversers for Manual and Autobrakes

• In summary, the deceleration from reverse thrust is always additive when using manual brakes, whether on a dry or a slippery runway.

• Conversely, the deceleration from reverse thrust may be additive when using autobrakes, depending on the autobrake setting and the runway conditions.

• The Advisory landing distance data contained in the Boeing QRH includes the effect of reverse thrust for Max Manual braking and for Autobrakes (whether additive or not).

• As a reminder, reverse thrust becomes the most effective deceleration device as runway conditions deteriorate.

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The End

David

Text Box

The following is a quote from a Boeing article, Landing on Slippery Runways Guide, Airliner/Oct-Dec 1992 Quote: The total decelerating force available on a dry runway is quite large, approximately .5g deceleration capability. This means the total stopping force available on a 500,000 pound Model 747 is 250,000 pounds, or 45,000 pound for a 90,000 pound Model 737. At high ground speeds approcxiamately 35% - 55% of the total force available is provided by drag and thrust reversers and 45% - 65% is provided by the brakes. At lower speeds, the brakes provide 80% - 95% of the total decelerating capability. On wet runways the total stopping force available is less than on dry runways due to the reduced braking effectiveness. The reversers and speedbrakes become more important since they now represent a larger protion of the total force capability. Wet runway braking capability is smallest at high speeds and increases as speed decreases. With the speedbrakes deployed, the drag and reversers furnish 50% - 80% of the high speed stopping force, whereas the brakes furnish 70% - 95% of the low speed stopping force. Overall, the wet runway stopping capability is 50% - 80% of the dry runway capability. Failing to extend the spoilers on a wet runway reduces the stopping capability by an additional 20% - 30%.

Boeing Slippery Runways Notes

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