Precision Straight Edges
		for Checking Airport Runway Smoothness
	Rough Pavement is Hazardous to Aircraft
	The evolution of the straight edge for testing pavement has roots in highway construction where smoothness affects the ride quality. Cars have suspension systems that dampen road roughness. Aircraft suspension systems are not designed to absorb pavement roughness but to absorb the energy of landing impact. This means that most of the available strut stroke is already used up when the aircraft is on the ground.
	Runway pavement surface irregularities may cause vibrations in the cockpit that make controls difficult to manipulate. Pavement profile irregularities can also cause increased stress and premature failure of critical airplane components. Uneven pavement can reduce braking capacity as the airplane responds to vertical acceleration.
	FAA Pavement Roughness Specifications
	The FAA advisory circular AC 150/5370-10D specifies the "Standards for Construction of Airports" for P-401 Plant Mix Bituminous Pavements and P-501 Portland Cement Concrete Pavement. The acceptance criterion for determining runway smoothness is determined using the following standard per section 501.52.E.3 page P-501-31. "As soon as the concrete has hardened sufficiently, the pavement surface shall be tested in the transverse direction with a 16-foot straightedge or other specified device. Surface smoothness deviations shall not exceed 1/4 inch from a 16-foot straightedge at any location, including placement along and spanning any pavement joint or edge." The FAA also expects both newly constructed or rehabilitated pavement to meet this standard. The ICAO Tolerable Limit is 3-mm (0.12-inch) in 3-meters (9.8 feet). There are also a few regional specifications that hold deviation as low as 1/8 inch in 16-foot for paving and seal coating.
	FAA Smoothness < 1/4 inch in 16 foot
	Straight Edges are a Better Choice than Profilers
	The FAA allows several other electronic systems that use various types of profilers that use indicators to simulate the results of the 16-foot straight edge. Per FAA specifications from - AC 150/5370-10D P-501-31 "Use of the profilograph to measure pavement smoothness is optional and will be approved on a case-by-case basis. Use of a profilometer may not be practical for all construction."
	Although these profilers are initially much more expensive than a typical 16-foot straight edge, they claim to reduce costs over the long term. Using a physical straight edge is also sometimes criticized as being manpower intensive. However the projected savings of profiling devices can quickly disappear if customers argue over the accuracy of the test.
	The statistical graphical result from a profilograph is not always easy for customers to understand. These unnecessary disputes cause manpower savings to evaporate with extra meetings, additional tests and delayed payments are considered. Electronic profilers also require maintenance and careful use for repeatable results. If normal industrial standards were applied profilers would also require periodic calibration to a NIST reference. The complexity of these profiler systems usually require highly trained personnel where even a minimum wage employee can operate a straight edge. Roughness is much easier for customers to see by just looking at the gap between a 16-foot straight edge and the runway. The gap can easily be quantified by using tapered shim gauges marked with the size and recorded. The straight edge test results are accurate and very hard to dispute unless the straight edge used was not straight.
	Screeds are not Good Straight Edges
	The majority of asphalt and concrete contractors often use a "Screed" as a straight edge to check paved surfaces for roughness. Screeds were designed to work concrete and should not be used as a straight edge because their straightness is unknown. New screeds taken out of the box failed to meet any reasonable quality standard for a measurement tool.
			After checking (4) new 16-foot aluminum screeds from two leading suppliers, each were found to have major flatness errors by as much as 0.069-inch. The screeds were also twisted and deflected by as much as 0.056-inch when end supported. These inspection results show that a contractor using a typical screed to check pavement roughness could be using up almost half of the 0.25-inch tolerance specified by the FAA with the combined 0.12-inch screed error. Good pavement could be rejected by a customer because a contractor used one of these typical screeds for runway inspection.
	Straight Edge Specifications for Runway Inspection
	Have you ever noticed how most other companies never mention what sort of quality they provide with their cheap straight edges? As of November 2009, the FAA has no specified certification requirement for the 16-foot straight edge as required in AC 150/5370-10D. The FAA does however require calibration standards for other aircraft inspection tooling as specified in AC 145-9 (4-12, p.53) that include procedures and records required for repair stations. Although these FAA standards were not specifically designed for runway maintenance tooling, they embody good practices that are common to ISO/DIN standards found in industry. It also seems reasonable that a typical industrial gauge specification of 10% of the tolerance range would be an appropriate standard for the FAA straight edge quality. Using this criterion, the standard flatness and straightness tolerance for a FAA 16-foot straight edge should be 0.025-inch. Since some regional and international specifications use a tighter 1/8-inch standard for checking pavement, a more universal tolerance for a 16-foot straight edge might hold flatness at 0.012-inch. To meet normal industrial gauge standards, this 16-foot straight edge should be traceable to a NIST surface, include a serial number and certification. This precision straight edge should be re-inspected at specified intervals to assure that it has not been damaged and record or fix any errors found.
	Straight Edge Tolerance = 0.012" error in 16 foot
	Straight Edges Designed for Contractor Use
	A 16-foot straight edge used for checking pavement must have a wide enough base to stand on the edge without out falling over. The unit must not rust or be susceptible to damage with normal use.
	The straight edge must be light enough for one person to lift and handle. Steel is too heavy because a 16-foot x 3.5-inch x 1.5-inch straight edge weighs 285 pounds and certainly exceeds NIOSH lifting limitations as published in 94-110. Aluminum extruded alloys offer light weight, but the extrusion process can twist the beam and the soft edge nicks easily. Bonding a continuous stainless steel edge to an aluminum box extrusion substrate provides durable non-rusting working edge. Adding aluminum location pads to the opposite side allows precision grinding each side of the 16-foot length to remove any surface errors on the precision straight edge. Handholds are then bonded to the sides of straight edge to make the unit easy to grip. A serial number is marked on the unit for permanent identification.
	Next the straight edge is checked against a precision NIST qualified granite surface plate and the deviation is recorded on a certificate of inspection that references the serial number. The completed precision straight edge fits inside a 4-inch PVC pipe for shipping that can also be mounted on top of a truck rack for easy access at the job site.
	New "PaveFlat" - Straight Edge for Runway Smoothness Inspection
	Overall Size: 16-feet x 3.6-inch x 1.5-inch Stainless Working Edge: 192-inches x 1.25-inches Straight Edge Weight: 33 lbs. Flatness and Straightness: 0.012-inch maximum deviation over 192 inches Serial number marked on unit for traceability Certificate of inspection included Shipped in a PVC tube Cost: $1,980 each. Freight additional - FOB Springfield, OH Shipping Weight with tube: 67 lbs. Delivery 1 week
	Re-certification of 16-foot straight edges
	No grinding or rework on the unit. Your straight edge is inspected for flatness and straightness Serial number used on unit for traceability Certificate of inspection completed Cost: $290 each Delivery: 1 week
	Re-work and Recertification of 16-foot straight edges
	Units must have enough stock for regrinding Units will be rejected is error is too great to remove Flatness and Straightness: 0.012-inch maximum deviation over 192 inches Serial number used on unit for traceability Certificate of inspection included Cost: $960 each Delivery: 2 weeks
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		For More Information Contact:
		Mike Petsch & Associates, Inc.		Phone (937)438-8031
		P.O. Box 24246		Fax (937)438-8035
		Dayton, OH 45424		Email petsch@juno.com
				PaveFlat.com