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The FAA proposed a new AD affecting certain Bombardier Inc. CL-600-2B16 (Challenger 604) aircraft following reports of cracks on the lower wing plank at the rear spar. The agency estimates the inspections would require up to 24 labor hours per aircraft at $85 per hour, for an estimated cost of $2,040 per aircraft. Maintenance providers would need to incorporate the inspections into scheduled visits and document compliance as part of the aircraft's maintenance history. Operators would inspect the lower wing plank rear spar web and lower flange, the forward surface of the pressure bulkhead frame under the stiffeners and the vertical stabilizer front spar-to-frame attachment fitting lower legs. Any cracking found during those inspections would require corrective action. The FAA estimates 201 affected aircraft are on the U.S. registry but does not have definitive data to estimate repair costs for aircraft where cracking is found. By proposing inspections of multiple structural areas following reports of cracking, the agency is attempting to identify similar issues elsewhere in the fleet before they develop further. If adopted, Challenger 604 operators would need to incorporate the inspections into future maintenance planning and account for the associated aircraft downtime. What Operators Need to Know The Challenger 604 remains a popular choice for charter operations, corporate transportation and owner-flown trips. An inspection requiring up to 24 labor hours can keep an aircraft down for days, depending on shop scheduling and workload. The AOG cost, not the $2,040 inspection bill, is what operators should budget for first.