- Unpaved Surface: A surface, intended for aircraft operations, composed of unbound or natural materials. Unpaved surfaces may include gravel, coral, sand, clay, hard packed soil mixtures, grass, turf or sod. (Note: Unpaved surfaces have also been referred to within the aviation industry as Unimproved Runways.)
- Unpaved Runway: A runway pavement constructed with an unpaved surface.
- Unprepared Surface: Any naturally occurring surface used as a runway that has not been altered by man.
Source: Transport Canada
Unpaved runway surfaces can be subject to significant variations in their strength and surface characteristics because of climatic effects and the effects of aircraft operations, according to guidance to aircraft operators and aerodromes from Transport Canada in December 2017. Unpaved runways can achieve their design strength and surface characteristics when maintained properly and not subject to excessive moisture. Gravel surfaces deteriorate with time and under repeated traffic loadings. The most common defects occurring with gravel surfaces are ruts, frost heaves, depressions, potholes, soft spots and loss of aggregates. Periodic grading, compaction and addition of new material are required to maintain the integrity of the gravel surface and to ensure the safe operation of aircraft.
Unpaved runway surfaces are typically non-homogeneous in composition and may contain various types of soils. Soil classification is used to predict the probable behaviour of soils under the influence of frost and moisture. The California bearing ratio (CBR) provides a measure of the ability of an unpaved surface to resist shearing under aircraft loads. CBR is the ratio of the load bearing capability of a given sample of soil compared to that of crushed limestone. The CBR of a given soil test is expressed as a percentage ranging from 0% to 100% or a whole number ranging from 0 to 100. CBR should be considered an index of runway surface strength as opposed to an absolute or true value of shear strength, because of the dependence of the CBR value on the measuring device used.
Weather and Wear Effects
Conditions of excessive moisture, such as those found during heavy precipitation, poor drainage or spring thaw can result in a significant degradation in runway surface strength. The degradation in surface strength may be enough to limit or completely restrict operational use.
During periods of extended and deep frost, unpaved surfaces such as gravel runways can have strength characteristics similar to those of paved hard surfaces. Operational experience has indicated that two weeks of ambient temperatures of –20°C or lower may be necessary for an unpaved runway to achieve strength similar to a paved hard surfaced runway. Once frozen solid, the runway will remain in this state, until ambient temperatures increase to above freezing. It is important to know however that while the frozen unpaved runway may have the same surface strength as a paved hard surfaced runway, it is necessary to ensure that the runway also meets all requirements including smoothness criteria. The surface characteristics should also be considered to ensure the required aircraft braking performance is met.
The following factors can adversely affect unpaved runways:
- Loss of material resulting in bare spots and sub-grade material appearing on the surface;
- Accumulation of loose, non-cohesive aggregates on the surface because of material segregation;
- Formation of ruts in wheel paths;
- Persistence of damp or wet areas because of poor surface drainage;
- Soft areas during spring thaw or wet conditions;
- Differential heaving or depressions because of frost action;
- Runway roughness or longitudinal unevenness (waviness); and
- Vegetation growth.
Effects on Aircraft Performance
- Increased takeoff distance due to the increased rolling resistance (caused by the deflection of the surface under the load of the aircraft).
- Increased stopping distance due to reduced braking performance.
- Increased accelerated stop distances (in case of rejected take off) due to the factors mentioned above.
- Degraded handling on the ground (during take-off and landing roll as well as ground manoeurving). The use of nosewheel steering may be necessary for improved handling, or in some cases prohibited if incompatible with unpaved surfaces.
- Procedures and equipment necessary to protect an aircraft may also have an adverse effect on takeoff and landing performance. The procedures usually limit pilot intervention and most protection equipment (e.g. shields, deflectors, etc.) impacts aircraft aerodynamics and weight.
Protection of aircraft components is necessary so that operators can safetly use aircraft on unpaved surfaces. Operators typically achieve this through special procedures and equipment, e.g.:
- Gradual application of thrust or power to minimize the ingestion of materials by engines or damage to propellers;
- Limited or prohibited use of reverse thrust;
- Configuration of bleed air systems to minimize dust ingestion;
- Reducing tire pressure without changing the aeroplane weight, which results in the redistribution of the wheel load over a wider area thus reducing runway surface deflection and tire rolling resistance. This may be limited, however, by the tire design and the necessity to avoid excessive deflection of the tire under load;
- Reduction of aircraft weights if operating under reduced tire pressures;
- Aircraft modification with oversize, floatation or balloon type low pressure tires for operations on soft unpaved surfaces; and
- Installation of protective systems (e.g. shields, deflectors, and filters, engine intake vortex dissipators, abrasion-resistant finishes, etc.). These protect the aircraft from hazards caused by the impingement and ingestion of stones, dust and debris.
In Canada, any modifications having a significant or appreciable effect on the aeroplane require an Aircraft Certification approval, in the form of a Supplemental Type Certificate (STC) or an equivalent approval document.
A supplement to the AFM, to provide the appropriate limitations, procedures and performance information (or data) for unpaved surface operations is normally required as part of a certification approval. The AFM supplement should also provide a surface definition to identify the characteristics of unpaved runway surfaces from which the aircraft has been certified to operate.
Any alterations of tire pressure, or types of tires used should be considered to be major modifications to an aircraft. These changes should undergo an aircraft type certification approval process.
Systems such as anti-skid and nose-wheel steering should be specifically evaluated for unpaved runway operations to determine and identify any handling differences.
Many small aeroplanes and older large aeroplanes do not have aircraft certification approvals or flight manual information for unpaved runway operations. Some manufacturers have provided unapproved information, often called manufacturer’s data as guidance for operations on specific unpaved surfaces. Transport Canada found that many air operators do establish specific operational procedures for unpaved runway operations to protect their aeroplanes from any adverse effects, but they may not account for the degraded aeroplane performance.