Runway macrotexture vs microtexture: what sets wet friction and hydroplaning from taxi to take-off speed

Macrotexture and microtexture are two scales of runway surface texture with different jobs when the surface is wet: macrotexture mainly sets how fast wet friction falls as ground speed rises, and microtexture mainly sets how high it starts. The International Civil Aviation Organization (ICAO) Airport Services Manual Part II, Pavement Surface Conditions (Doc 9137, 4th edition, 2002) sets out that split, and NASA TN D-2056 (November 1963) gives the dynamic hydroplaning speed relation (ICAO, 2002) (NASA, 1963). This is engineering background, not operational guidance: decisions to operate, restrict or close a runway belong to the aerodrome operator under its State's rules.

Key facts

• ICAO Doc 9137 Part II (4th edition, 2002) says macrotexture mainly sets the slope of the wet friction-speed curve and microtexture mainly sets its level.
• Macrotexture mainly counters dynamic aquaplaning and microtexture mainly counters viscous aquaplaning, according to ICAO Doc 9137 Part II section 2.3.1.
• Mean texture depth (MTD, ASTM E965 sand patch) and mean profile depth (MPD, ASTM E1845 and ISO 13473-1) are different quantities that need a transformation between them.
• The NASA relation V_p = 9√p gives a dynamic hydroplaning spin-down speed from tyre pressure alone, with no texture term, and its constant is rounded or fitted.
• FAA AC 150/5320-12C describes continuous friction measuring equipment run at 40 mph (65 km/h) and 60 mph (95 km/h) with a uniform 1 mm (0.04 in) water depth, and a complete survey uses both speeds.
• All friction-speed curves and sand-patch inputs in this guide are synthetic, and the hydroplaning figures are not operational speeds.

What is the difference between macrotexture and microtexture?

Macrotexture is the coarse texture of the aggregate arrangement, or of applied texture such as grooving, that Doc 9137 describes as mainly draining bulk water (section 2.3.1). Microtexture is the texture of the individual aggregate pieces. Doc 9137 describes it as felt but not directly measured, and as the scale working through thin films (ICAO, 2002).

Aquaplaning, also called hydroplaning, is the loss of tyre contact because water supports the tyre. Dynamic aquaplaning depends on the inertia of the water; as little as 0.5 mm of standing water has been found sufficient to support it, a finding and not a required depth (Doc 9137, 1.5.10). Viscous aquaplaning is a thin-film effect on very smooth surfaces. Macrotexture mainly counters the dynamic type and microtexture the viscous type, so macrotexture alone shows only general trends (ICAO, 2002).

How do macrotexture and microtexture compare side by side?

	Macrotexture	Microtexture
Scale	Aggregate arrangement; grooving	Individual aggregate pieces
What it mainly sets	The slope of the wet friction-speed curve	The level of wet friction
Aquaplaning it counters	Dynamic	Viscous
How it is measured	At rest, by a patch method or a profile (mean texture depth, MTD, or mean profile depth, MPD)	Doc 9137 (2002): felt but not directly measured; in practice inferred from low-speed friction

Doc 9137 states that the slope of the wet friction-speed curve is primarily affected by macrotexture and its level mainly by microtexture (1.5.15-1.5.16, 3.2.9) (ICAO, 2002). "Skid resistance" is the road term for wet friction, and "sand patch" is the common international name for the ASTM E965 volumetric test, which now uses glass beads. Road practice makes the same split: the South African road manual TMH 13 Part E (Committee Draft Final, May 2016) says macrotexture mainly governs how quickly skid resistance falls with speed (section E.2.4.1) (COTO, 2016f). See texture and skid resistance on South African roads and the guide on IRI, rut depth and texture depth.

Narrated film · 2:51

Texture, friction and speed

A texture cross-section with a water film, synthetic friction-speed curves for two macrotextures, and the 9√p hydroplaning relation built term by term.

Read the film transcript

Two wet runways can match at low speed, so why do they part company as the aircraft speeds up? The answer is two scales of texture, and what a hydroplaning formula leaves out. Macrotexture is the coarse texture of the aggregate arrangement, or of grooving, and it drains bulk water. Microtexture is the texture of the individual aggregate pieces, working through thin water films. ICAO Document nine one three seven says macrotexture mainly sets the slope of the wet friction curve, and microtexture mainly sets its level. Spreading material gives mean texture depth, a profile gives mean profile depth, and they are not the same quantity. The spread method gives texture depth, T D, equals four V, divided by pi D squared. V is the volume spread, in cubic millimetres, and D is the mean patch diameter, in millimetres. The material forms a thin disc, so volume equals pi D squared over four times depth, which rearranges to the equation. With a synthetic twenty-five thousand cubic millimetres on a patch one hundred eighty millimetres across, depth is zero point nine eight millimetres. Now friction: F of S equals F sixty, times the exponential of sixty minus S, over S p. F sixty is friction at the sixty kilometre per hour reference, and sets the level, mainly through microtexture. S is speed in kilometres per hour, and S p is a speed constant that sets the slope, mainly through macrotexture. This shape is empirical, chosen for illustration, and every curve here is synthetic. Surface A has an S p of eighty, and falls from zero point seven five two at sixty-five kilometres per hour to zero point five one seven at ninety-five. Surface B has the same level but an S p of two hundred, and keeps zero point six seven two. Surface C starts lower with B's slope, so its ratio between the two test speeds matches, zero point eight six one. That is why the FAA says a complete survey uses both test speeds. Now hydroplaning: V p equals nine times the square root of p, in knots and pounds per square inch. V p is the speed at which the rotating wheel spins down, and p is tyre inflation pressure. The nine is a rounded constant from the NASA report; balancing fluid lift against load gives eight point six three. At two hundred pounds per square inch the root is fourteen point one four two, so V p is one hundred twenty-seven point three knots, sixty-five point five metres per second. Against pressure, the speed climbs from ninety knots at one hundred pounds per square inch to one hundred thirty-three point five at two hundred twenty. Texture appears nowhere in this formula, and spin-up can occur up to thirteen knots lower. These are not operational speeds, and the correlation data did not reach the highest pressures. Read ICAO Document nine one three seven, the FAA circular, and ASTM. Macrotexture sets how fast wet friction falls, microtexture sets where it starts, and nine root p ignores both.

How is each scale measured, and are MTD and MPD the same?

No. Mean texture depth (MTD) comes from a volumetric patch: a known volume of material is spread into the surface cavities and divided by the area covered (ASTM E965-15(2024), volumetric technique) (ASTM, 2024a). Mean profile depth (MPD) is calculated from a measured profile (ASTM E1845-23; ISO 13473-1:2019) (ASTM, 2023a) (ISO, 2019). Each needs a transformation to estimate the other, and European Union Aviation Safety Agency (EASA) guidance refers to ISO 13473-1 for MPD (CS-ADR-DSN GM1 ADR-DSN.B.191(o)) (EASA, 2025). No coefficients are printed here.

EASA CS ADR-DSN.B.090 (Issue 7, 16 May 2025) sets out an average texture depth of a new surface of not less than 1.0 mm, and ICAO Annex 14 recommends a minimum for new surfaces; the hub guide on thresholds shows which document says what (EASA, 2025).

The formula, term by term: sand patch texture depth

Equation 1: TD = 4V / (π D²)

symbol	what it is in words	unit	where its value comes from	typical range
TD	mean texture depth of the patch	mm	calculated	synthetic here
V	volume of material spread	mm³ (1 mL = 1,000 mm³)	measuring cylinder	synthetic here
D	mean diameter of the circular patch	mm	measured across the patch	synthetic here

Where it comes from: geometry, not a fit. The spread material forms a thin disc of diameter D and mean depth TD, so V = π (D/2)² × TD, which rearranges to Equation 1.

Worked numbers (synthetic, calc/sand_patch_texture_depth.out.txt): with V = 25,000 mm³ and D = 180 mm, the area is π × 180² / 4 = 25,447 mm², so TD = 25,000 / 25,447 = 0.98 mm. Because TD varies as 1/D², a 10 mm error in D moves it about 11 % (1.10 mm at 170 mm; 0.88 mm at 190 mm); see repeatability and uncertainty.

What it means and where it stops: the result is an average depth at rest, not MPD. ASTM E965 governs the method. It assumes the material fills the cavities level with the peaks, which porous and grooved surfaces strain.

Which speeds and water depth do friction tests use?

Continuous friction measuring equipment (CFME) measures friction along the pavement with a smooth-tread tyre and self-wetting water. Doc 9137 describes self-wetting at a water depth of at least 1 mm (3.2.5), and the US Federal Aviation Administration (FAA) AC 150/5320-12C (Skid Resistant Airport Pavement Surfaces, base text 18 March 1997) has approved CFME run at 40 mph (65 km/h) and 60 mph (95 km/h) with a uniform water depth of 1 mm (0.04 in); a complete survey uses both speeds (paragraphs 3-16 and 3-17) (ICAO, 2002) (FAA, 1997).

As one published value, Table 3-1 (criteria in use in some States) lists a minimum friction level of 0.42 for a Mu-meter with a type A tyre at 70 kPa, at 65 km/h with 1.0 mm of water, an old-edition figure quoted for its test conditions; the limits are linked from the hub. The runway condition code comes from a matrix in ICAO's PANS-Aerodromes (Doc 9981) (ICAO, 2020), not a friction device. ASTM E1960 is the practice to read for how a friction index is calculated (ASTM, 2023b).

How does wet friction change with speed, and what sets the slope?

Wet friction falls as speed rises. Doc 9137 describes macrotexture as mainly setting the slope of that curve and microtexture as mainly setting its level (1.5.15-1.5.16, 3.2.9) (ICAO, 2002).

The formula, term by term: a common speed-gradient form

Equation 2: F(S) = F60 × exp((60 − S) / Sp)

symbol	what it is in words	unit	where its value comes from	typical range
F(S)	wet friction at speed S	none	calculated; measured by CFME in practice	synthetic
F60	friction at the 60 km/h reference slip speed	none	sets the level, mainly microtexture	synthetic 0.60, 0.80
S	slip speed in the IFI speed-gradient form; the synthetic curves here use travel speed for illustration only	km/h	an input; a fixed-slip CFME runs at only a fraction of its travel speed	65, 95; 250 extrapolated
Sp	speed constant	km/h	sets the slope, mainly macrotexture	synthetic 80, 200

Where it comes from: it is empirical, not derived from physics. Assume friction loses the same fraction per km/h, dF/dS = −F / Sp; integrating from the reference speed gives Equation 2. The loss per km/h shrinks as F shrinks, a shape chosen for illustration. ASTM E1960 is where to look for standard coefficients; Doc 9137 sections 1.5.15-1.5.16 and 3.2.9 describe the slope this constant represents. Doc 9137 does not use Sp or the exponential.

Worked numbers (all synthetic, calc/friction_speed_curves.out.txt): surface A has F60 = 0.80 and Sp = 80 km/h, so F(95) = 0.80 × exp(−35 / 80) = 0.80 × 0.6456 = 0.517. Surface B, with the same F60 but Sp = 200 km/h, gives F(65) = 0.780 and F(95) = 0.672 against 0.752 and 0.517 for A. Surface C, with F60 = 0.60 and Sp = 200 km/h, gives 0.585 and 0.504. The ratio F(95) / F(65) equals exp(−30 / Sp), so it is 0.687 for A and 0.861 for B and C: slope alone.

What it means and where it stops: two surfaces with similar friction at 65 km/h can part company at 95 km/h, so in this synthetic model two surfaces similar at the lower speed separate at the higher speed. The FAA statement that a complete survey uses both speeds carries the practical point. At a synthetic 250 km/h (69.4 m/s) the formula gives 0.074 for A and 0.309 for B, a point extrapolated beyond any test speed and neither a CFME slip speed nor a free-rolling condition, since a free-rolling aircraft wheel has almost no slip; it illustrates slope versus level only. Real curves need not stay exponential, and the model omits water depth and tyre wear.

What does the hydroplaning-speed formula include?

The best-known dynamic hydroplaning relation is the NASA spin-down speed of a rotating tyre. Texture is not in it.

The formula, term by term: dynamic hydroplaning speed

Equation 3: V_p = 9 √p (V_p in kt, p in psi)

symbol	what it is in words	unit	where its value comes from	typical range
V_p	ground speed at which the rotating wheel spins down	m/s (kt)	calculated	90.0 to 133.5 kt here
p	tyre inflation pressure	kPa (psi)	tyre data; an input only	100 to 220 psi, synthetic
F_v	vertical load on the tyre	N (lb)	tyre load; used in the derivation only	not tabulated
A_G	gross footprint area	m² (in²)	tyre footprint; used in the derivation only	not tabulated
C_L	lift coefficient	none	assumed about 0.7 in TN D-2056	about 0.7
ρ	fluid density	kg/m³	taken as water	about 1,000
9	rounded or fitted constant	kt per √psi	TN D-2056 equation (4)	first-principles 8.63

Where it comes from: fluid lift on the footprint is ½ C_L ρ A_G V², and it must support the vertical load F_v. With F_v / A_G ≈ p, a fluid density near water (1,000 kg/m³) and C_L ≈ 0.7, V = √(2p / (C_L ρ)), which is 4.438 m/s per √psi, or 8.63 kt per √psi. The report prints 9 in equation (4), a rounded or fitted constant, never exact. The metric form 6.35√P km/h, or 1.763√P m/s with P in kPa, is only a unit conversion of that constant (NASA, 1963).

Worked numbers (calc/hydroplaning_speed.out.txt; pressures are inputs only and no aircraft type is named): 689.5 kPa (100 psi) gives 46.3 m/s (90.0 kt), 1,034.2 kPa (150 psi) gives 56.7 m/s (110.2 kt), 1,379.0 kPa (200 psi) gives 65.5 m/s (127.3 kt) and 1,516.8 kPa (220 psi) gives 68.7 m/s (133.5 kt). For 200 psi, √200 = 14.142, so V_p = 9 × 14.142 = 127.3 kt × 0.514444 = 65.5 m/s. These are not operational speeds; hydroplaning risk for a specific aircraft belongs to the manufacturer, the operator and the flight documentation.

What it means and where it stops: the formula holds for smooth and closed-pattern tyres on water deeper than the tread can clear, and it ignores texture. The correlation data did not reach the 200 and 220 psi inputs; see TN D-2056 Fig. 4. Spin-up occurred as much as 13 kt below the spin-down speed (TN D-2056, citing TN D-1376), so the range is wider in landing and rejected take-off than in take-off. Applied to the 200 psi example, that bound would put spin-up as low as about 58.8 m/s (114.3 kt), an arithmetic illustration outside the tested pressures, not a prediction (NASA, 1963). Doc 9137 section 1.5.10 sets out the conditions it describes. The common misreading treats 9√p as a speed at which a runway becomes unsafe.

Frequently asked

What is the difference between macrotexture and microtexture on a runway?

ICAO Doc 9137 Part II (Pavement Surface Conditions, 4th edition, 2002) describes macrotexture as the coarse texture of the aggregate arrangement or of grooving, which mainly drains bulk water, and microtexture as the texture of individual aggregate pieces, which mainly acts on thin water films. Macrotexture mainly sets the slope of the wet friction-speed curve and microtexture mainly sets its level.

Are MTD and MPD the same texture measurement?

No. Mean texture depth (MTD) comes from a volumetric patch such as the ASTM E965 sand patch, while mean profile depth (MPD) is calculated from a measured profile under ASTM E1845 and ISO 13473-1. They are different quantities and need a transformation between them, so a report should name which one it states and never place them in one column.

Does the 9√p hydroplaning formula include runway texture?

No. The NASA TN D-2056 relation V_p = 9√p, with V_p in knots and p tyre pressure in psi, gives the spin-down speed of a rotating tyre for dynamic hydroplaning on a smooth or closed-pattern tyre in deep water. It ignores texture, its constant is rounded or fitted, and its results are not operational speeds for any aircraft.

At what speeds is runway friction tested?

FAA AC 150/5320-12C (base text 18 March 1997) describes approved continuous friction measuring equipment run at 40 mph (65 km/h) and 60 mph (95 km/h) with a uniform 1 mm (0.04 in) water depth, and a complete survey uses both speeds. ICAO Doc 9137 Part II section 3.2.5 describes self-wetting at a water depth of at least 1 mm.

How much water can support dynamic aquaplaning?

ICAO Doc 9137 Part II section 1.5.10 records that as little as 0.5 mm of standing water has been found sufficient to support dynamic aquaplaning. It is a finding, not a required depth, and the critical speed also depends on tyre pressure. Decisions about a specific runway belong to the aerodrome operator under its State's rules.

Where this connects

The hub guide on runway surface thresholds sets Doc 9137, FAA AC 150/5320-12C and EASA CS-ADR-DSN side by side. How to survey runway roughness on the centreline and wheel paths covers the profile that texture sits on, from runway survey to rehabilitation decision uses both, and Boeing Bump Index vs IRI vs straightedge covers roughness measures.

References

1. ICAO (2002). Airport Services Manual, Part II, Pavement Surface Conditions (Doc 9137). 4th edition, 2002. Original source.Sections 1.5.10, 1.5.15-1.5.16, 2.3.1, 3.2.5, 3.2.9 and Table 3-1
2. FAA (1997). AC 150/5320-12C, Measurement, Construction, and Maintenance of Skid Resistant Airport Pavement Surfaces. Base 18 Mar 1997; Change 9 dated 29 May 2025 (replaces App 4); Active. Original source.2-2; 2-15 to 2-22; 3-3 and Table 3-1; 3-16; 3-17; 3-19 and Table 3-2; App 4 (cite from 150-5320-12c-chg9.pdf)
3. EASA (2025). Certification Specifications and Guidance Material for Aerodrome Design (CS-ADR-DSN). Issue 7, 16 May 2025 (ED Decision 2025/004/R; applicable 24 May 2025; PDF corrected 16 Jun 2025). Original source.CS ADR-DSN.B.090 (a)-(d); GM1 ADR-DSN.B.090(a); GM1 ADR-DSN.B.191 (m)-(p)
4. ASTM (2024a). Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique (ASTM E965). E965-15(2024). Original source.Scope 1.1
5. ASTM (2023a). Standard Practice for Calculating Pavement Macrotexture Mean Profile Depth (ASTM E1845). E1845-23. Original source.Scope 1.1-1.3, Note 1
6. ASTM (2023b). Standard Practice for Calculating International Friction Index of a Pavement Surface (ASTM E1960). E1960-07(2023). Original source.Scope 1.1
7. ISO (2019). ISO 13473-1:2019, Characterization of pavement texture by use of surface profiles, Part 1: Determination of mean profile depth. 2019 (Ed. 2), confirmed 2024. Original source.Cited by EASA GM1 ADR-DSN.B.191(o)
8. ICAO (2020). Procedures for Air Navigation Services, Aerodromes (PANS-Aerodromes, Doc 9981). 3rd edition, 2020 (consolidates Amendment 3, applicable 5 Nov 2020); Amendment 5 applicable 27 Nov 2025 (OLS provisions 21 Nov 2030). Original source.Part II Chapter 2 (runway condition report, applicable 4 Nov 2021); table numbers unverified against the amended text
9. NASA (1963). Phenomena of Pneumatic Tire Hydroplaning, NASA TN D-2056 (W.B. Horne, R.C. Dreher). November 1963. Original source.Equations (3)-(5) and the assumptions under equation (5); Fig. 4; Hysteresis paragraph, p. 12
10. COTO (2016f). TMH 13: Automated Road Condition Assessments. Part E: Skid Resistance and Texture. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide

Use the manual edition and project requirements that apply to your work. Worked examples use synthetic values and are not site-specific design advice.