Aviation Topic of the Week
By Michael Oxner, December 21, 2003


Introduction
Previous Week's Topic
Following Week's Topic
Aviation in Canada Blog
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This week's topic:
Canadian Runway Friction Index

Following up to last week's topic, we'll slide into Canadian Runway Friction Index, or CRFI. I promised you a lot on this topic, and there is a lot. Let's get right to it.

Introduction
Application
How it is Measured
Ranges of Values
    Contaminant Table
When it Cannot be Measured
Tables
    Landing Distances
    Crosswind Component

Introduction

Last week, we looked at Aircraft Movement Surface Condition Reports. These are meant to be the plain language description of the runway condition. What good does that do by itself? How does 40% compact snow affect an aircraft on landing? We need a standardized method of determining what the effect of braking is given a certain type of runway contamination. Enter the Canadian Runway Friction Index. Old hands will remember when it was once called James Brake Index, or JBI. In fact, it's still known as that in many areas of the world. This is just a Canadian name for the same thing. For reference, the section in the AIP this week is AIR 1.6.

Application

The values obtained when measuring CRFI are used by pilots to determine extra runway length required and if a runway is even useable. Most aircraft (if not all) are supplied with charts that determine runway lengths required for landing. These charts usually have factors for considering wind, slope, temperature, and humidity possibly including others. One thing they don't consider is runway contamination. This is where CRFI come in. Through real world, extensive experimentation, tables have been produced to determine, based on CRFI measurements, the runway length required given a certain value when comparing to bare and dry pavement. For example, a B727 may be able to use an 6,000 foot runway when it's bare and dry, and even when it's bare and wet. Throw some slush, ice or snow on the runway, and suddenly that same 6,000 runway may be way too short. Since the values are, by definition, a reflection of runway friction, the CRFI also gives a pilot a "heads up" about maximum acceptable crosswind component.

Also, something that some folks don't consider that real world pilots should is runway friction's effect on take-offs. If all goes well on a take-off, the aircraft accelerates, rotates and lifts off. This may not require much runway, especially for a light aircraft. For heavier aircraft, and high performance aircraft, another factor becomes more important than for light aircraft: the runway length required for an aborted take-off. While this should be considered by any pilot, many pilots fly light aircraft out of airports with excessively long runways for their types, and therefore this consideration generally gets glossed over, even forgotten about. A pilot must consider the length of runway required to accelerate to take-off speed and abort the take-off successfully without overrunning the end of the runway. Since this manoeuver requires deceleration, runway friction plays an important role in this aspect as well. For a high performance aircraft like a jet, even water on the runway can make the difference between go and no-go since water alone acts like a lubricant, affecting braking.

There are so many variables to account for that accelerate-stop distances required are not currently published. The data obtained is considered valid for landing and for crosswinds, but they have yet to publish a table for take-offs. This, then, must be considered seriously by the pilot prior to take-off.

How it is Measured

CRFI is taken by a vehicle, often no more than a pickup truck, that is fitted with a decelerometer. This device measures the deceleration forces acting on the vehicle when the brakes are applied. The vehicle is run along the runway at approximately 1,000 foot intervals within 30 feet either side of the runway centerline. A long runway could take a long time to measure. The readings taken are then averaged and this is the value that is stated in a CRFI report.

Ranges of Values

The values range from 0, the theoretical value for absolutely no friction, to 1, the theoretical value for "maximum decelerating capability of the vehicle on a dry surface". According to the AIP, a value of 0.8 would be roughly equivalent to that experienced on bare and dry pavement, although discussions with an airport duty manager who used to record these values indicated that 0.7 is more likely to be the measurement in such conditions. As mentioned in last week's topic, a value less than 0.40 will be reported in an AMSCR. For some anecdotal information, the lowest value I can recall seeing was 0.12, and that's pretty low. Many aircraft don't even bother to descend to approach an airport when they hear values that low, since it almost invariably means the runway isn't long enough. At least, that's the way it is in the maritime provinces where our longest runway is 8,800 feet. Have a look at the table below to get an idea of what to expect from various types of runway contamination. This was scanned directly from the AIP, and reflects averages for typical contaminants and their effects on a runway's friction.

RSC-CRFI

When it Cannot be Measured

Because certain conditions provide false or inaccurate CRFI values, CRFI is not always reported with an AMSCR. "Technical limitations" of the decelerometer equipment are cited as the reason in the AIP. A CRFI will not be measured and will not be reported when any of the following runway conditions exist:
  1. The runway surface is simply wet with no other type of contamination present
  2. There is a layer of slush on the runway surface with no other type of contamination present
  3. There is loose snow on the runway surface exceeding 1 inch in depth
Also, a CRFI may not be included despite the presence of contaminants that would normally have a report issued. For example, a runway with 60% bare and dry pavement with 40% ice patches may not have a CRFI value taken since the runway average would reflect high friction values not experienced on the ice.

Tables

The moment you've been waiting for, the tables. I've reproduced these as accurately as I can, but the possibility exists that I didn't find an error when I made these. To that end, it is very important that you refer to official documents instead of this page when CRFI information is required when lives are on the line. These tables come with some notes, too, which must be considered. Again, I urge you to review official documents to make sure you understand everything about them if you're planning to use CRFI tables in the real world. This particular part of the page is provided for information's sake in the hobbyist's world only.

Essentially, the notes applicable to the tables include the following information. Since the tables were provided based on statistical calculations, they are rated statistically, too. They say these are at a level of confidence of 95%, meaning that 19 times out of 20 the distances will be enough. The distances in the tables do not consider the braking effects of thrust reversers for jets, or discing or reverse pitch of props, either. Aircraft equipped with such abilities may well require less than the tables mention. The information is left uncorrected for these aspects considering that there are situations in which a pilot might find himself where the additional braking abilities are not available (system failure, engine out, etc).

In all cases, the distances are calculated based on standard pilot techniques for minimum landing distances from 50 feet, including a stabilized approach at Vref on a glideslope of 3° to 50 feet or lower, a firm touchdown, minimum time to lowering the nose to the runway, minimum time to deployment of lift dumping devices (spoilers) and application of brakes at sustained maximum antiskid ability until actually stopped. The tables below include methods of reading corrected distances required whether your Aircraft Flight Manual expresses landing performance in distances required (enter from the left hand side) or landing field length (enter from the right hand side). Make sure you know which method is in use by the AFM. In the case the AFM uses landing field lengths, the table below would have you enter the table from the right hand side, and consider landing distance required divided by 0.6 (60%) for jet aircraft, and 0.7 (70%) for turboprops.

Reported Canadian Runway Friction Index (CRFI)
Landing Distance (Feet)
Bare and Dry
Unfactored
 0.60
 0.55
 0.50
 0.45
 0.40
 0.35
 0.30
 0.27
 0.25
 0.22
 0.20
 0.18
 Landing Field Length (Feet)
Bare and Dry
 Landing Field Length (Feet)
Bare and Dry
Recommended Landing Distances (No Discing/Reverse Thrust)
 60% Factor
 70% Factor
1,800
 3,120
 3,200
 3,300
 3,410
 3,540
 3,700
 3,900
 4,040
 4,150
 4,330
 4,470
 4,620
 3,000
 2,571
2,000
 3,480
 3,580
 3,690
 3,830
 3,980
 4,170
 4,410
 4,570
 4,700
 4,910
 5,070
 5,250
 3,333
 2,857
2,200
 3,720
 3,830
 3,960
 4,110
 4,280
 4,500
 4,750
 4,940
 5,080
 5,310
 5,490
 5,700
 3,667
 3,143
2,400
 4,100
 4,230
 4,370
 4,540
 4,740
 4,980
 5,260
 5,470
 5,620
 5,880
 6,080
 6,300
 4,000
 3,429
2,600
 4,450
 4,590
 4,750
 4,940
 5,160
 5,420
 5,740
 5,960
 6,130
 6,410
 6,630
 6,870
 4,333
 3,714
2,800
 4,760
 4,910
 5,090
 5,290
 5,530
 5,810
 6,150
 6,390
 6,570
 6,880
 7,110
 7,360
 4,667
 4,000
3,000
 5,070
 5,240
 5,430
 5,650
 5,910
 6,220
 6,590
 6,860
 7,060
 7,390
 7,640
 7,920
 5,000
 4,286
3,200
 5,450
 5,630
 5,840
 6,090
 6,370
 6,720
 7,130
 7,420
 7,640
 8,010
 8,290
 8,600
 5,333
 4,571
3,400
 5,740
 5,940
 6,170
 6,430
 6,740
 7,110
 7,550
 7,870
 8,100
 8,500
 8,800
 9,130
 5,667
 4,857
3,600
 6,050
 6,260
 6,500
 6,780
 7,120
 7,510
 7,990
 8,330
 8,580
 9,000
 9,320
 9,680
 6,000
 5,143
3,800
 6,340
 6,570
 6,830
 7,130
 7,480
 7,900
 8,410
 8,770
 9,040
 9,490
 9,840
 10,220
 6,333
 5,429
4,000
 6,550
 6,780
 7,050
 7,370
 7,730
 8,170
 8,700
 9,080
 9,360
 9,830
 10,180
 10,580
 6,667
 5,714

The AIP has a second, similar table which accounts for the braking ability of discing for props and reverse thrust for jets. Since this page isn't intended to be an all-encompassing document, I won't reproduce that here. This table above is presented solely for information. I have highlighted one spot in the table above in red for the sake of an example. If you're aircraft normally requires a runway landing distance of 3,600 feet in bare and dry conditions, a runway contaminated with compacted snow, according to the table above and the earlier table regarding runway contaminants and their effects on CRFI, a landing at Halifax International if the CRFI was 0.22 or less would be dicey, since the required runway length is now 9,000 feet, and the longest runway there is 8,800. As mentioned earlier, reverse thrust would cut that distance down, supposing everything finctioned normally when you needed it.

Now let's have a look at the crosswind component chart out of the AIP. I scanned this one, since it was easier to reproduce that way. I will also use the example right out of the AIP for demonstration purposes.

CRFI-Crosswind

Example:
                CYOW CRFI RWY 07/25 -4 .3 9301191200
                TWR Wind 110° 20KT

The wind is 40° off the runway heading and produces a headwind component of 15 knots and a crosswind component of 13 knots. The recommended minimum CRFI for a 13 knot crosswind component is .35, A take-off or landing with a CRFI of .3 could result in uncontrollable drifting and yawing. [Values are directly out of the AIP's example, though I think they are a little bit off]




I told you there would be enough for CRFI as a topic of its own. It's a lengthy one, but it you're flying the big iron in the real world, it's something to consider. Especially if you're from down south where the snow doesn't fly and you happen to be flying up north for a change. The effects can be quite unexpected, and it's too late to consider this when you're actually on the runway. Once again, I'm always willing to accept feedback. My e-mail address is moxner@nbnet.nb.ca. Many thanks to those who have taken the time to write already. Next week, due to the ever present pressures of the holiday season, I will not be producing a Weekly Topic. I'll be taking a little vacation. I hope you enjoy the holidays as much as I intend to. Here are some holiday wishes in languages of readers who have contacted me so far. Except for the Hawaiian one at the end. I just like the sound of that one.
MERRY CHRISTMAS!(English)
JOYEUX NOËL!(French)
BUON NATALE!(Italian)
 FELIZ NAVIDAD!(Spanish)
 KALA CHRISTOUYENNA!(Greek)
 FROHE WEIHNACHTEN!(German)
MELE KALIKIMAKA!(Hawaiian)