Understanding aircraft climb requirements can be a challenge, as this is one of the areas where the world of engineers blends with the world of operators. Depending on the geographical region, both the airworthiness and the operational requirements can vary significantly. Nevertheless, there are certain aspects which apply on a global scale. This article aims to summarize the concept of climb requirements while providing reference to the regulatory guidance material. The focus lies on transport category aircraft operation in accordance with ICAO PANS-OPS and EU-OPS.
If there is a nice mountain in the climb-out sector, it is obvious that somehow there must be a set of rules how to deal with that…
Figure 1: Typical takeoff flight path with obstacles
…as it turns out, there are many rules, even if there is no mountain…
The categories
Are we talking about the aircraft itself or about it’s operation? This seemingly obvious question should be at the start of every climb performance discussion, as the two tend to get mixed-up every now and then. The importance of getting this right becomes obvious as soon as we look at the underlying conditions for each case: There are significant differences…
Figure 2: Extract of associated conditions for climb requirements
The aircraft – airworthiness
Certification specifications for transport category aircraft stipulate certain climb capabilities. These can be thought of a “minimum performance” to get a type-certificate. Two things are important here: Climb capability is described as a performance requirement for the aircraft. There is no reference to the “ground” as such, so wind has no effect [1]. (Notice that this article strictly deals with climb requirements, not takeoff distance). Further, the climb capability required has to be demonstrated with one engine inoperative (OEI) [1]. CS-25 divides the takeoff flight path into different segments, which are defined by specific events [1]. The numbering as such is not binding, the gradients however are. The location, where the aircraft reaches the screen height (35 ft) above the takeoff surface, is sometimes called “reference zero” (R0) [1].
Figure 3: Takeoff climb requirements acc. CS-25
The operations – PANS-OPS and EU-OPS
Things change when we look at operating an aircraft at a certain location. There are procedures to follow, either instrument departures (e.g. SID’s) or visual departures [2]. Climb gradients here can be driven by factors such as airspace structure, noise abatement, navaid coverage or obstacles [2] [3] [4]. With respect to SID climb requirements, it is important to realize, that these assume “normal operation” [2], so all engines operating. As soon as the aircraft encounters “abnormal operation” (e.g. OEI), the SID gradients no longer apply, neither do noise abatement requirements. The operator is responsible to develop so-called “contingency procedures” for such a case [2] [4]. Important here: the obstacle clearance requirements still apply for abnormal operation, in fact this is probably the limiting case in most situations [4].
Obstacle clearance – What are gross and net gradients?
Operating rules require the net takeoff flight path to clear the applicable obstacles by 35ft [4]. Aircraft manufacturers are required to determine the net gradient, by reducing the actually achieved (gross) climb gradient by a given percentage [1] [5]. So the aircraft will perform better than the provided net gradient, this is important to keep in mind when interpreting obstacle clearance requirements. A reduction in climb gradient may be translated into an equivalent reduction in acceleration capability for the acceleration segment [1].
Figure 4: Reduction of climb gradient acc. CS-25
Is gross always better than net?
Surprisingly not. This special case is more related to regulatory interpretation than to physics: By definition, the climb path starts at 35 ft above the takeoff surface, at the end of the takeoff distance required [1]. 35 ft being the screen height. Now, if the runway is wet or contaminated, the screen height is reduced to 15 ft, but the definition of the net flight path does not change. Therefore, if taking off from a wet or contaminated runway, and experiencing an engine failure at Vef, the aircraft is completely legal to cross a close-in obstacle by only 15 ft…[4] (See pitfalls below).
Figure 5: Special case for wet or contaminated runway
Obstacle clearance – Which obstacles?
Obviously not every aircraft has to clear Mt. Everest. There is a specific “takeoff funnel”, defined by EU-OPS CAT.POL.A.210. Depending on the aircraft’s navigation performance, wingspan and the amount of track change, different lateral limits are given as indicated below:
Figure 6: Lateral obstacle consideration acc. EU-OPS
Pitfalls
When analyzing climb capabilities, it can be very hard to find the actual climb performance of an aircraft for normal operation, as the airworthiness requirements mandate publication of OEI data only [1]. Obviously it is safe to say that AEO performance will be better than OEI, but just how much?
Meeting the SID gradient with OEI sounds conservative, but often it is unrealistic and even if it is achieved, not everything is automatically “alright”. The SID design gradient is not adjusted for “close-in” obstacles (<200ft) [2], so if there are any of those, caution should be exercised.
When a contingency procedure has been developed, it might be worth thinking about what happens, if an abnormal situation develops after the point where the contingency procedure deviates from the SID…
Figure 7: Typical initial climb situation
This article is by no means complete, but hopefully it highlights the critical aspects and encourages the interested reader to consult the regulatory guidance material, even if things seem obvious.
This is what performance consideration is all about: Frequent calculations for rare events.
Revision/20190516
References
[1] EASA, CS-25, amdt 22
[2] ICAO, PANS-OPS, doc 8168, volume 1, 5th edition
[3] ICAO, PANS-OPS, doc 8168, volume2, 6th edition
[4] EU, regulation 965/2012, revision 12
[5] Swatton, Aircraft performance theory for pilots, 2005 edition, Blackwell Science