1. Metric units apply to new airfield construction and where practical modification to existing airfields and heliports,
as discussed in paragraph 1.4.4.
2. The criteria in this manual are based on aircraft specific requirements and are not direct conversions from inch-
pound (English) dimensions. Inch-pound units are included only as a reference to the previous standard.
3. Airfield and heliport imaginary surfaces and safe wingtip clearance dimensions are shown as a direct conversion
from inch-pound to SI units.
4.13. Obstructions and Airfield Airspace Criteria. If the imaginary surface around a rotary-wing
runway, helipad, landing lane, and hoverpoint is penetrated by manmade or natural objects as defined in
Attachment 6, the penetrating object is an obstruction. Determination and dealing with obstructions are
further discussed in Attachment 6.