To G450CPT, Aeroncaman, Zeffy, and aterpster...
Whether it is TERPS or PANS-OPS, IFR departure procedures are developed based on normal airplane operation and performance. Contingency procedures addressing engine failure and non-normal operations are addressed outside of these criteria. It states this fact this both documents. The FAA has stated the same in AC 120-91 and now again in a new series of performance planning videos that will be made available soon.
The 1st and 2nd segment OEI climb gradient data provided in the AFM does not allow for independent assessment of obstacle clearance without application with the close-in and distance takeoff flight path/obstacle clearance charts that are also published in the AFM. Only in the case of the final segment OEI climb gradient may a “rise v. run” analysis of obstacle clearance be accomplished. The reason for this are many, but they were eloquently explained by Mr. Rob Root from Boeing’s performance engineering & operations training group at this week’s Transport Airplane Performance Planning Working Group’s workshop at the NBAA 2012 convention. Simply put, limiting the takeoff weight to that which allows an OEI climb gradient (e.g. OEI 2nd segment) to meet or exceed the SID climb gradient is not an acceptable solution to the OEI takeoff obstacle analysis.
The OEI takeoff climb gradient and OEI flight path performance data that is published in the AFM is used for the takeoff obstacle avoidance analysis required by 121.189(d) and 135.379(d), respectively. The procedure used to conduct this analysis is contained in AC 120-91. This OEI takeoff obstacle avoidance analysis is not a pilot function. The FAA fully expects this analysis to be performed by a trained performance engineer or furnished through a performance engineer services provider, both of which should have the requisite training and airport & obstacle data resources to complete this analysis. FAA has now stated this fact in the previously mentioned videos.
One more item, the OEI takeoff obstacle clearance requirements for US part 121 and part 135 operators applies to any takeoff regardless of whether the conditions are IMC or VMC, or the whether the airplane is operating under instrument flight rules (IFR) or visual flight rules (VFR). OEI visual obstacle avoidance is permitted under the limitations prescribed in AC 120-91, which typically requires that the operator identify the critical obstacle(s) to be avoided and provide any instructions that are to be used in the event of an engine failure. It is not acceptable to just say, “since it’s VMC, we will just see and avoid any obstacles after an engine failure”.
Regarding compliance with a SID climb gradient, there is not an easy question to answer. Depending on the aircraft manufacturer, all-engine-operating (AEO) climb performance data may be available. However, this AEO climb data is not simply a “climb gradient”. Rather, there is an AEO flight path that must be assessed. To illustrate what this AEO path might look like we can say that the climb profile begins with a V2+10 climb to 1000’ AAE followed by an acceleration to 250 KIAS at a nominal rate of climb (e.g. 500 FPM). The profile continues at climb speed of 250 KIAS to 10,000 ft. MSL. During this segment of the profile, the data must account for climb degradations resulting from thrust lapse rate and TAS increase. Once above 10,000 ft., the climb speed increases to enroute climb speed, which for many airplanes using COST INDEX will be a variable speed as altitude increases. The same climb degradations must be considered in the climb above 10,000 ft. The entire profile must also account for wind factors, which may switch around 180 degrees or more in the climb out.
So as you can see, the AEO climb profile does not equate to the constant “gradient” the presented on the SID chart. Only very sophisticated programs, for example Boeing’s Climb Out Program, are capable of analyzing the AEO climb profile against a SID climb gradient requirement. Many airlines use the BCOP and Airbus’s equivalent tool to assess SIDs at various airports. The output of these programs may furnish weight limits to the crew for various conditions or the airline’s engineering department may run the analysis for a wide-ranging set of variables for their fleet to see if there are any AEO performance issues with a particular SID. If performance issues arise, they will address them at time of dispatch or will simply not allow permit their pilots to use the SID. In some cases, they will remove the SID for the nav-database.
Even with the FAA review of SID climb gradients greater than 500’/NM, there still can be performance issues with some transport airplanes. FAA has reported issues with A340s on SIDs at DFW and PHX and with A319s at PHL in meeting the altitude restrictions published on SIDs and ergo, the AEO climb gradient since these gradients were established to meet these ATC –required altitude restrictions. No offense to Airbus airplanes.
For those airplanes without performance tools like the BCOP, it really comes down to your knowledge of the airplane’s capabilities and your best estimate of whether you can make the climb gradient that published on the SID. The OEI case is different since as part 121 or part 135 operator, the pilot/operator must depart with an established takeoff obstacle avoidance procedure. That procedure may follow the SID’s track or may differ.
For US pilots, the FAA will be providing additional information to pilots and operators on this subject. The information will be available from the FAA website and from other websites. More to come soon.
Best regards,
Rich Boll
Co-Chair, Transport Airplane Performance Planning Working Group.