From certification to a safe dispatch
Connect the certification basis, operating class and the margins that make a flight dispatchable.
Which performance rules apply?
Certification describes what the design has demonstrated. Operating rules describe how that capability may be used on a particular flight.
Start with two separate questions
Certification describes what the design has demonstrated. Operating rules describe how that capability may be used on a particular flight.
CS-23 concerns normal-category aeroplanes; modern CS-23 uses performance-based requirements and levels. Earlier small-aeroplane manuals often use the prescriptive amendment basis under which that type was certified. CS-25 concerns large transport-category aeroplanes, with extensive engine-failure and performance demonstrations. Do not silently apply a new CS-23 amendment to an older approved manual.
Choose the CAT performance class
Under Part-CAT, class A includes all turbojet-powered aeroplanes and multi-engine turboprops with MOPSC above 9 or maximum take-off mass above 5,700 kg. Class B comprises propeller-driven aeroplanes with MOPSC 9 or less and maximum take-off mass 5,700 kg or less. Class C comprises reciprocating-engine aeroplanes above either of those thresholds. MOPSC is the maximum operational passenger seating configuration.
A class-A operation is not synonymous with a jet. A light twin is not guaranteed a positive climb after engine failure. The safety level comes from certification capability combined with operational restrictions. A structurally legal mass may therefore be too high for the runway, climb, obstacles or destination.
Why do demonstrated and permitted performance differ?
Measured performance comes from controlled flight and ground tests. Corrections produce representative gross performance for the stated conditions and procedures; it is not the best result a particularly skilled crew once achieved. Net performance applies prescribed conservatism to represent the performance that can safely be planned.
Measured, gross, net
Measured performance comes from controlled flight and ground tests. Corrections produce representative gross performance for the stated conditions and procedures; it is not the best result a particularly skilled crew once achieved. Net performance applies prescribed conservatism to represent the performance that can safely be planned.
A distance requirement becomes longer when a factor is applied; the permitted mass from a fixed runway becomes lower. A gross climb gradient becomes smaller when the prescribed gradient decrement is subtracted. For descent, a conservative net path must avoid optimistic obstacle clearance; check the sign and definition rather than blindly subtracting a number.
A margin has a purpose
Margins account for operational scatter, event likelihood and differences between test conditions and service. A rare engine failure and a routine landing do not use one interchangeable multiplier. Some AFM charts already contain the required allowance. Applying it twice gives a wrong answer, even if the answer looks conservative.
For CS-25 take-off paths the gross-to-net gradient reductions are 0.8, 0.9 and 1.0 percentage points for two-, three- and four-engined aircraft. This is subtraction of percentage points, not multiplication by 0.992. Landing operational factors belong to their own rule and stage of assessment.
Can you make the performance decision?
Four original questions. Work from the stated conditions and review the reasoning.