What is Longitudinal Grade?

The longitudinal grade of a pavement is its slope along the direction of travel, in percent. A grade of 1% means the surface rises or falls 1 m over 100 m. FAA AC 150/5300-13B defines surface gradient as “the rate of ascent or descent of an airport surface”. EASA and ICAO call the same quantity “slope”, and the FAA also writes “grade” and “gradient”.

On a runway, the direction of travel is along the centreline. ICAO computes the average runway slope as the difference between the highest and lowest centreline elevation, divided by the runway length. The word longitudinal gives the direction. The grade itself is a height change per horizontal distance.

Longitudinal grade and cross-slope

Cross-slope, also called transverse slope, runs across the pavement. It drains water off the surface toward the edges. EASA says the runway surface should be cambered, which means it slopes down from the centreline to both edges. A single crossfall is the exception. The whole surface then slopes from one side to the other, in the direction of the wind that most often comes with rain.

Pavement	Longitudinal grade	Cross-slope
Runway	1.25% (code number 4), 1.5% (code number 3) or 2% (code number 1 or 2), along any portion	1% to 1.5% (code letter C to F) or 1% to 2% (code letter A or B)
Taxiway	1.5% (code letter C to F) or 3% (code letter A or B)	Up to 1.5% (code letter C to F) or up to 2% (code letter A or B)
Aircraft stand	Up to 1% in any direction	Up to 1% in any direction

These are EASA values from CS-ADR-DSN.B.060, B.080, D.265, D.280 and E.360. The runway longitudinal limits depend on the code number, and the cross-slope limits depend on the code letter. The runway cross-slope has a minimum of 1%. At runway or taxiway intersections, flatter slopes may be necessary. The FAA also limits the runway cross-slope. AC 150/5300-13B Table 3-6 gives 1.0% to 2.0% for approach categories A and B, and 1.0% to 1.5% for categories C, D and E.

Water runs along the combined longitudinal and transverse slope. ICAO Annex 14 Attachment A and EASA GM1 ADR-DSN.B.191 say that this combined slope is the path of the runoff. Transverse grooves shorten it. EASA GM1 ADR-DSN.B.080 says that an adequate combination of longitudinal and transverse slopes drains the surface water.

Why it matters

The FAA states that an upsloping runway impedes acceleration and gives a longer ground run on takeoff. A downsloping runway affects deceleration and gives a longer ground rollout on landing. The gradient standards also protect:

• operation within aircraft structural limits
• surface water drainage, to reduce the risk of hydroplaning
• the pilot’s line of sight
• ride smoothness

Runway limits in EASA and ICAO

CS-ADR-DSN.B.060 and ICAO Annex 14 Volume I, paragraphs 3.1.13 to 3.1.16, use the word “should”. In Annex 14 they are Recommendations, not Standards. They set three kinds of limit.

• Average slope. Divide the difference between the highest and lowest centreline elevation by the runway length. The result should not exceed 1% for code number 3 or 4, or 2% for code number 1 or 2.
• Slope along any portion. The slope should not exceed 1.25% for code number 4 and 1.5% for code number 3. For code number 1 or 2 it should not exceed 2%. In the first and last quarter of a code number 4 runway, it should not exceed 0.8%. The same 0.8% applies to the first and last quarter of a Category II or III precision approach runway of code number 3.
• Slope changes. A change between two consecutive slopes should not exceed 1.5% for code number 3 or 4. For code number 1 or 2 it should not exceed 2%. The transition should be a curved surface. The rate of change should not exceed these values:
  • code number 4: 0.1% per 30 m (radius 30,000 m)
  • code number 3: 0.2% per 30 m (radius 15,000 m)
  • code number 1 or 2: 0.4% per 30 m (radius 7,500 m)

A worked case shows what the rate means. A 1.5% slope change on a code number 3 runway is 7.5 steps of 0.2%. Each step takes 30 m, so the curve is at least 225 m long. This is the same as the radius of 15,000 m times the change of 0.015.

Successive curves should not sit close together. Annex 14, paragraph 3.1.18, sets a minimum distance between their points of intersection. Take the sum of the two slope changes in percent. Multiply it by 30,000 m (code number 4), 15,000 m (code number 3) or 5,000 m (code number 1 or 2). The distance should not be less than this value or 45 m, whichever is greater. EASA GM1 ADR-DSN.B.075 gives an example for code number 3. The slopes are +1%, then -0.5%, then +0.5%. The sum of the changes is 1.5% plus 1%, which is 2.5%. The distance is at least 15,000 m times 0.025, which is 375 m.

Where a slope change cannot be avoided, a pilot should see along the runway. CS-ADR-DSN.B.070 asks for an unobstructed line of sight between points above the runway for at least half its length. The height is 3 m for code letters C to F, 2 m for code letter B and 1.5 m for code letter A.

ICAO adds guidance for precision approach runways. Slope changes should be avoided or kept to a minimum on a rectangular area at least 300 m long before the threshold. The area is 120 m wide. Aeroplanes that make automatic landings use a radio altimeter over this ground. Where a slope change cannot be avoided, the rate of change between two consecutive slopes should not exceed 2% per 30 m. This is Annex 14 Attachment A, section 4.3. It is guidance, not a Standard.

Taxiway and apron limits

CS-ADR-DSN.D.265 says a taxiway slope should not exceed 1.5% for code letters C to F, or 3% for code letters A and B. CS-ADR-DSN.D.270 limits the rate of slope change to 1% per 30 m (radius 3,000 m) for code letters C to F. For code letters A and B the limit is 1% per 25 m (radius 2,500 m). The same specification adds a fallback. If these rates are not achieved, the curve should allow the safe operation of all aircraft in all weather conditions.

On an apron, CS-ADR-DSN.E.360 says the slopes should be enough to prevent water from collecting, and no more than drainage needs. On an aircraft stand the maximum slope is 1% in any direction. EASA GM1 ADR-DSN.E.360 says the design should consider the slope’s effect on an aircraft that brakes at the stand and starts to move for departure.

FAA standards

AC 150/5300-13B, paragraph 3.16.1, sets standards for runway centrelines by aircraft approach category.

Item	Categories A and B	Categories C, D and E
Maximum grade	2.0%	1.5%
Maximum grade change	2.0%	1.5%
Minimum vertical curve length	300 ft (91 m) per 1.0% of change	1,000 ft (305 m) per 1.0% of change
Minimum distance between points of vertical intersection	250 ft (76 m) times the sum of the grade changes in percent	1,000 ft (305 m) times the sum of the grade changes in percent

Vertical curves are parabolic. For categories A and B, no vertical curve is necessary when the grade change is less than 0.40%. A 1.5% grade change in categories C, D and E needs a curve of at least 1,500 ft (about 457 m).

For categories C, D and E, two limits apply within the lesser of two zones. The zones are the first and last quarter of the runway, and the first and last 2,500 ft (762 m). Grades above 0.8% are not acceptable there. Runway grade changes are not acceptable there either.

For taxiways, paragraph 4.14.1 sets a maximum longitudinal grade of 1.5%. On taxiways that serve only aircraft of 30,000 lb (13,605 kg) or less, the maximum is 2.0%. The maximum grade change is 3.0%. The minimum vertical curve length is 100 ft (30.5 m) for each 1.0% of change.

How longitudinal grade is measured

A photograph does not show a grade. A grade comes from surveyed elevations along the centreline, compared with the design profile. The FAA gives rules for one use of such a profile, the measurement of runway roughness. AC 150/5380-9 evaluates bumps on the longitudinal elevation profile of the runway. The FAA requires a survey interval of 0.25 m for this method. It recommends profiles along the centreline and at a lateral offset on each side that matches the aircraft that use the airport.

The same circular does not recommend inertial profilers that use highpass filtering for this work. The filtering changes the long wavelengths of the profile, which the Boeing Bump computation needs. Such profilers also make errors where the test vehicle accelerates or brakes near the thresholds.

No source in this check gives a survey method or an accuracy figure for drone-derived runway profiles, so this page states none.

How the grade changes in service

Settlement of the foundation soil can lower part of a pavement. The FAA PAVER manual calls such a low area a depression. It says a depression causes roughness. Filled with enough water, it can cause hydroplaning of aircraft. A depression changes the local grade and can leave ponded water on the runway. The manual says to measure the depth of a depression up to 3 m (10 ft) across with a straightedge laid over it.

On a concrete pavement, settlement shows as faulting. The PAVER manual for concrete airfields defines settlement or faulting as a difference of elevation at a joint or crack that upheaval or consolidation causes.

ICAO Annex 14 Attachment A, paragraph 5.5, says that deformation of a runway over time can form water pools. Pools as shallow as about 3 mm, in a place where landing aeroplanes meet them at high speed, can induce aquaplaning.

Grade, slope change and surface irregularity

A grade is a long, smooth feature. A surface irregularity is a short one. EASA GM1 ADR.OPS.C.010(b)(2) defines a surface irregularity as an isolated deviation in elevation that does not lie along a uniform slope. It gives limits by the length of the irregularity.

Length of irregularity	3 m	15 m	45 m
Acceptable height	2.9 cm	5.4 cm	8.5 cm
Tolerable height	3.9 cm	8.6 cm	13.6 cm
Excessive height	5.8 cm	10.8 cm	17 cm

Above the acceptable height, the operator should plan maintenance. Above the tolerable height, corrective action is mandatory. The runway may stay in service, but it should be repaired within a reasonable period. Above the excessive height, the area of the runway warrants closure. The values are guidance. The same table is in ICAO Annex 14 Attachment A, paragraph 5.3.

Construction tolerance

For new work, FAA AC 150/5370-10H, item P-152-2.13, sets a construction tolerance. The graded surface under a subbase or base must stay within 0.05 ft (15 mm) of the specified grade. The grade is measured on a 50 ft (15 m) grid. This is a construction tolerance, not a limit for a runway in service.

Item P-401-5.3 tests the finished asphalt with a 12 ft (3.7 m) straightedge. The variance must be less than 1/4 inch in 12 ft. The specification says not to make smoothness readings across grade changes or cross slope transitions.