Low Visibility at Night

Figure: Test aircraft, from Pavement Grooving and Traction Studies, page 121, figure 2.

Eddie Sez:

Every now and then, setting up for landing on a wet, short runway, I hear that we should consider the grooved runway to be dry. I have flown aircraft that allow this, but the Gulfstream does not. There isn't a lot of current research out there, most of it was done forty years ago. Aircraft manufacturers can take advantage of grooved runways in performance numbers if they wish, but they don't have to.

We all know intuitively that a grooved runway makes it easier to stop. But braking on a wet, grooved runway as easy as on a dry runway? No, but it is more complicated than that:

  • Landing distances on a wet, grooved runway are remarkably reduced almost to the point of being dry, until the water depth is greater than 0.1 inch or if there is any slush. At that point the distances are improved but not nearly as much.

  • Balanced field lengths are "essentially dry" for grooved runways in a "wet and puddled" condition. If the runway is slush covered, the advantages of runway grooves are only "slight."

Given that, I wonder if my previous aircraft manufacturer was giving us too much leeway to consider a wet, grooved runway to be dry for takeoff and landing. The key takeaway: if the grooved runway is wet with less than 0.1" depth, your takeoff performance is nearly as if the runway was dry. Your landing distance is improved, but not as dramatically. You still need to plan according to the manufacturer's performance charts, but know you may do better in actual practice.

What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.


NASA Wallops Station Tests

Figure: Effect of slush covered runway grooves on directional control of 990 aircraft, from Pavement Grooving and Traction Studies, page 63, figure 24.

The NASA Langley Research Center conducted tests to determine the affects of grooved runways on aircraft braking performance at the NASA Wallops Station in 1968. Flight tests were conducted using an Air Force F-4D aircraft and a NASA Convair 990 aircraft. The results were presented in a conference report that compiled 27 papers from this and other studies. It is dry and convoluted writing, 521 pages of it, but there is a lot to be gained from selective reading. A copy of the report is linked, and excepts follow.

While the study covered various topics, such as tire wear and runway drainage, only three of its focus areas are of interest to us here:

  1. Grooved runway impact on tire rolling resistance.

  2. Grooved runway impact on tire cornering ability.

  3. Grooved runway impact on stopping capability.

Test set up

[Pavement Grooving and Traction Studies]

Aircraft tire rolling resistance

Low Visibility at Night

Figure: Effect of runway wetness condition and surface configuration, from Pavement Grooving and Traction Studies, page 49, figure 2.

[Pavement Grooving and Traction Studies]

Aircraft tire cornering ability

Figure: Cornering force, from Pavement Grooving and Traction Studies, pages 50 - 53, figure 3.

[Pavement Grooving and Traction Studies]

  • [pg. 38] In this figure the cornering force obtained for each particular surface configuration is divided by the cornering force value obtained under the same conditions on the dry ungrooved surface. This value is then multiplied by 100 to express it as a percentage of dry ungrooved cornering force.

  • [pg. 38] In general, the data show that a greater percent of the dry ungrooved surface cornering force was obtained on the damp test surfaces as compared to that obtained for flooded conditions. For similar wetness conditions, the data obtained on the ungrooved test surfaces are significantly lower than the data obtained on the sawed-groove and flailed-groove test surfaces. However, the degradation in tire cornering force developed on the saw-groove test surfaces for flooded conditions is substantially less than that fro the damp surface condition, particularly at the higher, more critical speeds.

  • [pg. 38] The data in [the figure] show that under flooded conditions the sawed groove configuration of 1 in. x ¼ in. x ¼ in. maintained the greatest percent of dry ungrooved surface cornering force throughout the test speed range.

A grooved runway gives you a definite advantage on takeoff in either the stop or go scenario. If the takeoff is continued, this so-called cornering ability helps keep you within centerline tolerances. More about this: Technical / VMCG - Minimum Control Speed Ground.

Aircraft tire braking effectiveness

Figure: Braking effectiveness, from Pavement Grooving and Traction Studies, page 52, figure 4.

[Pavement Grooving and Traction Studies]

  • [pg. 39] In general, the locked-wheel friction coefficient data obtained on both the sawed- and flailed-groove surfaces are significantly higher than the data obtained on the ungrooved concrete test surface throughout the test speed range. However, as speed is increased, there is a rapid reduction in the locked-wheel friction coefficient obtained on the ungrooved and flailed groove test surfaces.

Braking Distances

Figure: Landing field lengths, from Pavement Grooving and Traction Studies, pages 109 and 110, figures 2 and 3.

[Pavement Grooving and Traction Studies]

  • [pg. 107] The present study of the effects of transverse runway grooving for the 990A and F-4D airplanes with regard to adverse-weather landing field lengths and balance take-off field lengths indicates the following conclusions:
    1. Transverse runway grooving effectively reduces landing field lengths under adverse weather conditions for a variety of runway surfaces.

Figure: Balanced field lengths, from Pavement Grooving and Traction Studies, pages 111 and 112, figures 6 and 7.

    1. Essentially dry balanced field take-off performance is attainable for grooved runway surfaces in a wet and puddled condition, since grooving increases the critical engine-failure speed to practically dry-surface values.

Figure: Balanced field lengths on slush, from Pavement Grooving and Traction Studies, page 113, figure 10.

    1. Only slight reductions in balanced field lengths are provided by grooving for take-off from slush-covered runways.

Regulatory

[14 CFR 25 §25.105(c)(1)(ii)] The takeoff data must be based on . . . At the option of the applicant, grooved or porous friction course wet, hard-surfaced runways.

[14 CFR 25 §25.109(d)] Accelerate-stop distance. . . . At the option of the applicant, a higher wet runway braking coefficient of friction may be used for runway surfaces that have been grooved or treated with a porous friction course material.

[14 CFR 25 §25.1533(3)] Additionally, at the option of the applicant, wet runway takeoff distances may be established for runway surfaces that have been grooved or treated with a porous friction course, and may be approved for use on runways where such surfaces have been designed constructed, and maintained in a manner acceptable to the Administrator.

I've flown aircraft that allowed pilots to consider wet grooved runways to be essentially dry. The Gulfstream 450 does not. The performance section of that AFM does not mention runway grooves at all. The only thing in our books on the subject appears in G-450-OIS-02:

[G-450-OIS-02, page 19] For landing operations on a wet, grooved runway, data in this OIS will be conservative.

This leads me to believe Gulfstream has not factored in grooved runways and wants you to use it as a safety pad. More about this: Procedures & Techniques / Braking Action.


References

14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation

Gulfstream G450 Operational Information Supplement, G450-OIS-02, Contaminated Runway Performance, Revision 1, August 3, 2011

Pavement Grooving and Traction Studies, NASA SP-5073, 1969