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06 May 2020

Airbus Consulting Services: Recommendations for pavement (storage area) preservation

A330-900 new MTOW first flight – Taxiing

Airbus' A330-900 weight variant – which benefits from a maximum take-off weight increase to 251 tonnes – begins its taxi to the Toulouse-Blagnac Airport runway ahead of the maiden flight

Runways and taxiways are designed for different purposes than aprons and parking areas. In particular they are not designed for accommodating static loads, therefore aircraft storage on those areas increases the risk of permanent deformations. It is estimated that 90% of aprons and aircraft parking areas world wide are made of concrete slabs (PCC) to minimize the risk of permanent deformation when they are subjected to static loads.

Recommendation for pavement (storage area) preservation

  • Aircraft storage area: It is recommended to prefer areas covered by Portland Cement Concrete (PCC)/Rigid pavements.
  • Flexible-type pavements (Asphalt Concrete coating) are not usually intended for parking areas.They are more sensitive than concrete paving to static loads and also more sensitive to high temperatures due to bitumen’s rheological properties
Aci Mapping

Base course strain 3D heatmap

Main Considerations

  • When considering where to park aircraft, make  sure that the pavement has adequate strength to accommodate the loads, and avoid positions close to seal cracks.
  • Check first ACN/PCN compatibility (or any other rating system) with aircraft operating weight. Note that overload operations can be  permitted up to +10% of the reported PCN, provided  that the pavement does not exhibit any sign of distress or failure.  “Overload” means  any load larger than that for which the pavement was intended to serve, or any significant increase in the number of load applications.
  • When possible, make the aircraft as light as possible with the most forward CG position permissible, without jeopardizing its resistance to wind. In those cases, it is preferable to moor the aircraft by adding concrete blocks (or steel  plates)  close to, and attached to the Nose and Main Landing Gears by means of a sling system, rather than by adding extra ballasts in the aircraft which may produce an aircraft ACN exceeding the reported PCN.
  • By doing so, aircraft mooring is ensured through additional loads which are not  added to the existing aircraft individual wheel loads. The weight of the  concrete blocks should be no more than 5 tons each, and the contact surface no less than one  square meter.  Interface between ground and blocks should  be provided by  an  intermediate material  ensuring  an adequate friction  between  each surface (in particular in wet  condition) and protecting  the pavement  surface integrity.
  • Airports are encouraged to perform a technical analysis before overloading pavements by  more  than 10%  relative to the reported pavement strength. According to the pavement data available, Linear Elastic Analysis (LEA) is recommended to obtain the best estimate of the additional stresses or strains that static aircraft will produce on pavement during the aircraft storage period.
Aci

Longitudinal strain profile at the bottom of base course

"Aircraft long time storage may be considered as an OVERLOAD if the storage area was not designed for this purpose. In those cases, pavement analysis should be performed."

 Bituminous materials have a viscoelastic behaviour and their modulus of elasticity depends of the temperature and the load frequency/speed (bituminous material rheological properties). Consequently, the slower the speed, the lower the (complex) modulus of elasticity and the higher the shear strain at the bottom of the bituminous layer. For  this reason, parking and aprons (where aircraft operate at low to very low speed) are usually made of concrete slabs. The strength of taxiways (moderate speed section) is greater than runways (except  in some cases on runway thresholds), but  lower than parking/aprons which are purposely designed for accommodating static aircraft loads.

It should then be understood that taxiways subjected to static aircraft loads are more sensitive to permanent deformation than those subjected to moderate or high speed loadings.

The A380 is sized to be compatible with taxiways, runways and parking aprons at airports around the world

A380 parked on taxiway

Airbus Best Practices for aircraft storage on taxiways and runways

  • Aircraft positioning: It is advised to offset the aircraft position from the pavement centreline (the optimum offset value being a function of the traffic regularly operating on the pavement), thus using pavement sections that are theoretically under-used by the regular traffic. The interactions with past traffic mix would therefore be reduced, and the impact on the pavement lifespan minimized.
  • Perform a complete pavement inspection before and after use as a storage area. Potential distresses may incude (but are  not limited to): Depressions under wheels on flexible pavements, ripples  and bumps, damage from fuel spillage or other hydrualic fluids (on those areas, protection should be provided by a solvent resistant surface), puncturing in case of inadequate pavement design.
  • Perform a daily routine inspection and when possible move the aircraft in case of signs of pavement distress or failure.
  • Clearance between aircraft should be sufficient to allow the repositioning of an aircraft in the event of surface distress or failure (greater than ICAO standards where possible).
  • For hot climate regions, note any significant increase of temperature which could affect the pavement surface. In those cases, it is recommended to deflate the tyres, but not more than tyre deflection as recommended by tyre manufacturers.
  • Note that deflated tyres reduce the normal force per unit area by spreading the wheel load over a larger contact area, but theoretically have no effect on friction. Indeed, friction is proportional to the normal force (Fs = μs x Fn) of the asphalt acting upon the aircraft tires. This force is equal to the weight which is distributed over each tire when the aircraft is on ground. This force can be stated as Pressure x Area. For a low tire pressure, the contact area is larger but the force per unit area is smaller and reciprocally. The friction force is therefore the same regardless of the tire contact area.

Summary

In any case, the return to service concerns both the aircraft and the pavement. If some pavement distresses have not been identified prior to the return to service, the risk will be transferred to the aircraft. In those cases, the required repairs to pavement would affect operations with consequential difficulties for the return to service and/or alter safety of operations.

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