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Braking Action - Winter Flying

by Darren Smith, CFII/MEI
October, 2007

  Decoder:  Braking Action | Weather Contractions | Abbreviations list | NOTAMs Contractions | Aircraft

Soon, it will start to snow in the Northern parts of the country.  For those of you who are sunbelt fliers (like myself) we rarely hear about braking action and decelerometers.  A recent question prompted me to do quite a bit of research on the topic and here are the results.

Want the complete reference guide?  Click the green thumbnail to the above right. 

To start off with, a few definitions are required...
Braking Action Term
More braking capability is available than is used in typical deceleration on a non-limiting runway (i.e., a runway with additional stopping distance available).  Directional control good.
  • Wet runway with a water depth of 1/8” or less
  • Compacted snow with OAT below -15ºC
  • ¾” or less of dry snow
0.40 & above
Medium to Good


0.36 – 0.39
Noticeably degraded braking condition. Expect and plan for a longer stopping distance such as might be expected on a packed or compacted snow–covered runway.  Effective directional control.
  • Compacted and sanded snow
  • Compacted snow with an OAT above -15ºC
  • Sanded ice
0.30 – 0.35
Medium to Poor


0.29 – 0.26
Very degraded braking condition (a potential for hydroplaning). Expect and plan for a significantly longer stopping distance such as might be expected on an ice-covered runway.  Directional control minimally effective.
  • Wet snow
  • Slush
  • Wet runway with a water depth more than 1/8”
  • Heavy rain
  • Ice
0.25 & below
Braking action is minimal to non-existent and/or directional control uncertain.
  • Wet Ice

How do we measure MU Values?

MU values are created by devices which measure runway friction or braking action capability.  Runway friction (braking action) measuring reports are to be expressed using the name of the FAA-approved device, followed by the “MU”(pronounced “mew”), followed by the reported values and the time of the measurement. Here are the types of approved devices and their NOTAM abbreviations:
  • BOW Bowmonk Decelerometer (Bowmonk Sales)
  • BRD Brakemeter-Dynometer
  • ERD Electronic Recording Decelerometer (Bowmonk)
  • GRT Griptester (Findlay, Irvine, LTD.)
  • MUM Mark 4 Mu Meter (Bison Instruments, Inc.)
  • RFT Runway friction tester (K.J. LAW Engineers, Inc.)
  • SFH Surface friction tester (high pressure tire) (Saab, Airport Surface Friction Tester AB)
  • SFL Surface friction tester (low pressure tire) (Saab, Airport Surface Friction Tester AB)
  • SKH Skiddometer (high pressure tire) (AEC, Airport Equipment Co.)
  • SKL Skiddometer (low pressure tire) (AEC, Airport Equipment Co.)
  • TAP Tapley Decelerometer (Tapley Sales)

How is braking action reported?

The NOTAM created could simply define braking action as provided by pilots using the runway.  A sample looks like this:

Example: !ANC ANC 1/19 BA FAIR WEF 0802061500
Translated:  At ANC airport, braking action for Runway 1/19 was reported as FAIR.  The report was effective February 6, 2008 at 1500UTC.

Airport management is responsible for observing and reporting the condition of a movement area.  This includes measuring runway friction with an FAA approved device.  With this device, MU values are created to describe each third of a runway.  A sample NOTAM looks like this:

Example: !DCA DCA 18 RFT MU 52/30/42 WEF 0712251000
Translated:  At DCA airport, Runway 18 was measured with an RFT and provided MU values of .52 (good braking) for the touchdown zone, .30 (medium braking) midfield, and .42 (good braking) for the rollout portion of the runway. The report was effective December 25, 2007 at 1000UTC.

Since airport management is responsible for observing and reporting the condition of a movement area, a variety of field conditions can be reported.  This includes reporting SNOW, ICE, SLUSH, and STANDING WATER on the runway.  There are 5 required elements for this kind of NOTAM:
  1. Affected runway
  2. Measurement - The depth is always expressed in terms of thin (less than 1/4 inch), 1/4 inch, 1/2 inch, and 1 inch.
  3. Coverage - A surface not completely covered should be described as having patches of snow, ice, etc. (PTCHY) or the absence of a described surface indicates the entire landing area.
  4. Type of coverage (SNOW, ICE, WATER) - see below, the 15 types of runway conditions.
  5. Time of observation 

The 15 types of runway conditions

  1. Snow.   (SN, LSR, PSR, WSR)
  2. Ice.   (IR)
  3. Snow and ice  (SIR)
  4. Slush.   (SLR)
  5. Water.   (WTR)
  6. Drifting or drifted snow - DRFT is used to describe one or more drifts. When the drifts are variable in depth, report the greater depth.
  7. Plowed/swept - PLW/swept are used when indicating that a portion of a surface has been plowed or swept and is either bare or has depth, coverage, and conditions different than the surrounding area. When known, the surrounding area items will be specified as RMNDR and listed after the plowed information. Plowed/swept is omitted when the entire runway has been plowed.
  8. Sanded, deiced
  9. Snowbanks - Snowbanks shall be assumed to be at the edge of a movement surface, or when plow/swept are used, at the edge of the plowed/swept area.
  10. Mud
  11. Frost
  12. Frost Heave
  13. Cracks
  14. Ruts
  15. Soft Edge
Sample runway condition NOTAMS:

Example: !MIV MIV 10/28 1/4 IN WSR WEF 0712251505
Translated:  At MIV airport, Runway 10/28 has 1/4 inch of wet snow on the runway (WSR) reported on December 25, 2007 at 1505UTC.

Example: !ANI ANI 10/28 THN LSR OVR 1 IN PSR WEF 0802091300
Translated:  At ANI airport, Runway 10/28 has thin (THN) loose snow on the runway (LSR) and over 1 inch of packed snow on the runway (PSR) reported on February 9, 2008 at 1300UTC.

Example: !DLG DLG 1/19 1 IN IR WEF 0708132145
Translated:  At DLG airport, Runway 1/19 has 1 inch ice on runway (IR) reported on August 13, 2007 at 2145UTC.

Example: !CLE CLE 1/2 IN WTR WEF 0802241700
Translated:  At CLE airport, a 1/2 inch of water reported on February 24, 2008 at 1700UTC.

Example: !ENA 1/021 ENA 1L/19R 4 IN LSR WEF 0801171315 
Translated:  At ENA airport, Runway 1L/19R has 4 inches loose snow on the runway (LSR) reported on Jan 17, 2008 at 1315 UTC.

 "Do not let yourself be forced into doing anything before you are ready." — Wilbur Wright 

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