Bombardier is formally seeking approval to operate its new CSeries twinjet at altitudes above 40,000ft, as the airframer approaches first flight of the prototype.
The manufacturer needs approval because US certification requirements technically prohibit such high-altitude flights owing to the potential exposure risk to passengers in the event of the fuselage being holed by uncontained engine failure.
This has forced airframers to request relief from the certification requirement in order to qualify modern aircraft to operate above 40,000ft - in recent years Boeing and Airbus have sought such exemption for the 787, A350 and A380, typically for operations at 43,000ft.
In its submission to the US FAA, Bombardier says no aircraft design with wing-mounted engines has been certified as complying fully with the requirements.
It is applying for approval for both the CS100 and CS300 to cruise at 41,000ft because the FAA rule is "not in the public's best interest" as it restricts the aircraft's operational flexibility.
The CSeries will use Pratt & Whitney PW1500G engines. Bombardier says it is "difficult to quantify" the risk of uncontained failure because it must resort to "second generation" data while the geared-fan powerplants are "third generation".
But it estimates the risk of such failure at high altitude at 2.8 x 10-8 per aircraft hour, and believes the likelihood for the CSeries is "lower" than similar previously-certificated transport aircraft with wing-mounted engines.