Greetings 400A,
I ran across this topic by accident. You raise some interesting questions. I wouldn’t be overly worried about the storm you may have created. It’s long overdue. Please allow me to take a few moments to comment on this post.
The newly revised AIM guidance for 5-2-8, released 27 August 2009, is intended to address these very questions. Misconceptions concerning expected airplane performance on TERPS-based procedures, SIDs and ODPs, were rising to a level where they needed to be addressed in the AIM. A year ago the effort was begun to address these concerns and with the help of AFS 410, we were able to make these changes. For more a detailed explanation, see the FAA/Industry Aeronautical Charting Forum agenda item history document on this particular AIM change:
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/acfipg/open/media/Hist%2008-01-279.pdf
With the publication of the AIM guidance, this matter will likely be closed at next month’s meeting. However, I remain troubled and concerned by the understanding of the OEI regulatory performance requirements by non-Part 121 operators, and particular by the Part 142 Training Organizations.
As you mentioned in your discussion, the performance requirements of TERPS (SIDs & ODPs) and the regulatory OEI performance requirements of Subpart I, Part 121 & Part 135 are two separate and independent subjects. Part 25 requires the aircraft manufacturers to provide the necessary performance data in the FAA-Approved AFM for the operator to show compliance with the operating rules contained in 91.605 and Subpart I, Part 121 & Part 135. TERPS performance requirements are not addressed by the operating rules; hence there is no requirement for the airframe OEM to provide all-engines operating performance data that can be used for TERPS performance compliance. However, there are efforts underway to address these requirements which are beyond the scope of the operating rules. See related ACF-IPG agenda item below:
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/acfipg/open/media/Hist%2098-01-197.pdf
On a related matter, additional guidance is provided in the 27 August 2009 AIM edition concerning compliance with climb gradients and altitude restrictions on ODPs and SIDs:
http://www.faa.gov/air_traffic/publications/ATPubs/AIM/CHG3AIM8-27-09.pdf
An ODP climb gradient is always established for obstacle or procedure design criteria purposes. A climb gradient or altitude restriction published on an ODP may not be canceled or amended lower by ATC. As noted above, ODP climb gradient requirements are based on normal, all-engines operating performance. SIDs are system enhancement procedures designed to facilitate air traffic clearance and traffic management. While assessed for obstacle clearance purposes per the TERPS criteria, climb gradients and altitude constraints may be published for air traffic separation/management purposes. These climb gradients and altitude constraints will be annotated “(ATC)” on future SIDs that are designed IAW FAA Order 8260.46D.
A climb gradient or altitude restriction published on a SID that is annotated (ATC) may be canceled by ATC at their discretion. On future SIDs, an (ATC) altitude restriction on the initial departure route will be accompanied by a second altitude restriction that is required for obstacle clearance purposes. This second altitude restriction is mandatory and cannot be amended by ATC.
The 27 August 2009 AIM change explains the differences between an ODP and SID climb gradient. An ODP climb gradient must always be complied with unless higher-than-standard takeoff minimums are published and the weather conditions at the time of takeoff meet or exceed those minimums. If the climb gradient is published without higher than standard takeoff minimums, then climb gradient is required, most likely for procedure design purposes. For an example, see the new RUUDY TWO RNAV SID for TEB runway 24. The initial climb gradient of 376 FPNM is required to support the turn to intercept the 260 track to WENTZ, which will occur at distance less than 2 NM from the departure end of runway 24. This climb gradient supports the need for the aircraft to be at least 400 ft above DER elevation before beginning a turn more than 15 degrees (see FAAO 8260.44).
91.175(f)(4) requires Part 121 and Part 135 operators departing IFR (not IMC, but rather on an IFR flight plan in either IMC or VMC conditions) to have a takeoff obstacle avoidance plan that meet the requirements of Subpart I, Part 121 or Part 135 as applicable:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=7e03cbb00617a7eecaaaf47605d76f4b&rgn=div8&view=text&node=14:2.0.1.3.10.2.6.42&idno=14
The Subpart I OEI takeoff obstacle clearance requirements are the same for both Part 121 and Part 135. They require the net takeoff flight path to clear obstacles by 35 ft and 200 ft laterally either side of flight track within the airport boundary, and by 300 ft laterally either side of flight track after passing the airport boundary. Subpart I also require the operator to account for effects of wind on the OEI flight track. AC 120-91, Airport Obstacle Analysis, provides guidance on operator compliance with this second requirement. The Obstacle Accountability Areas specified in the AC for straight-out and turning departures account for wind variations along the OEI lateral track without the need for specific wind accountability along the OEI flight track.
As noted in the AC, a Part 25 airplane’s OEI flight path is not linear, nor is it based on a single climb gradient value. Rather, the OEI flight profile is constructed of a series of flight path segments. The climb gradient charts provided for a Part 25 airplane provide for a value or “spot gradient” valid only at on particular point in the OEI climb. The 2nd segment climb gradient value may be the climb gradient attained at the point of landing gear retraction (e.g. Boeing & Dassault), 400 ft above the runway (e.g. Challenger 300), 1000 ft (e.g. Lear 45) or some other value. No two airframe OEM’s use the same method and will frequently vary with make and model.
The 2nd segment climb gradient value provides only a reference value for use with the takeoff flight path/obstacle charts. These flight path/obstacle charts are usually broken into close-in (1st & 2nd segment) and distant flight path charts. The flight path charts will usually contain some means of correction for effects of thrust lapse rate (thrust loss) through the OEI climb. Under no circumstances is it ever acceptable to use the 2nd segment climb gradient chart without reference to the flight path/obstacle charts. This is extrapolating data beyond the procedures found in the AFM, something the FAA very much frowns upon. Yet, I am well aware that this method is often taught at the Part 142 Learning Centers (who’s sign hangs over the door matters little).
Part 121 air carrier performance engineering departments or commercial performance engineering vendors (APG, Jeppesen OpsData, AeroData) use AFM takeoff flight path charts and performance data coupled with the “best available obstacle data“ (as required in AC 120-91) to verify OEI net takeoff flight path obstacle clearance that meet the requirements of Subpart I and in most cases meets the requirements of AC 120-91. If necessary to maximize takeoff weight, a special OEI procedure (EO procedure) may be provided. The EO procedures may follow a published ODP or SID, and operator’s special IFR departure procedure, e.g. the Eagle CO Cottonwood Special DP, or may be tailored EO procedure.
The OEI analysis does not always end at 1,500 ft. The OEI takeoff flight path per 25.111 ends at 1,500 ft, but AC 120-91 requires consideration beyond 1,500 ft into the enroute environment, a safe holding pattern, or return to the departure airport as appropriate. Also, Subpart I requires OEI obstacle clearance in the enroute phase of flight. The guiding principle of Subpart I is that, following an engine failure at or above V1, the airplane is assured obstacle clearance all the way to the destination an if necessary alternate airport (see 121.191/135.181).
TERPS criteria can form the basis of the OEI obstacle analysis, but as the AC 120-91 points out, the TERP climb gradient will be treated as a surface though which the airplane cannot penetrate from above. In other words, the OEI takeoff path must remain above the TERPS climb gradient surface or the OEI net takeoff flight path must remain 35 ft above the TERPS obstacle clearance surface (40:1 surface for a standard 200 FPNM CG). Further, there is a need to account for low, close-in obstacles which are not accounted for by the TERPS climb gradient. These are the obstacles found in the notes on ODPs or SIDS, or on the Jeppesen 10-4 OBST charts. These obstacles must be plotted on the AFM’s takeoff flight path/obstacle charts to verify obstacle clearance.
Since a TERPS climb gradient may be established for procedure criteria purposes as opposed for obstacle clearance purposes (see RUUDY TWO at TEB), using the TERPS climb gradient to meet Subpart I or 91.175(f)(4) takeoff obstacle clearance requirements may be unduly burdensome. The airport analysis method provides a much greater takeoff weight allowance.
There was much discussion on the FAA’s opinion on this subject. Rather say anything for the FAA, I thought you might like to hear it from the current manager of FAA’s AFS 410, Mr. Coby Johnson. The FAA produced an excellent training video hosted by Mr. Johnson for their inspector staff explaining AC 120-91, which may be viewed at this link:
http://videoontheweb.faa.gov/training/AFS-AC120-HS.asx
Please allow me to summarize a few key points:
I hope that this discussion was helpful in addressing this latest AIM change and recent FAR changes.
Best regards,
Rich Boll
I ran across this topic by accident. You raise some interesting questions. I wouldn’t be overly worried about the storm you may have created. It’s long overdue. Please allow me to take a few moments to comment on this post.
The newly revised AIM guidance for 5-2-8, released 27 August 2009, is intended to address these very questions. Misconceptions concerning expected airplane performance on TERPS-based procedures, SIDs and ODPs, were rising to a level where they needed to be addressed in the AIM. A year ago the effort was begun to address these concerns and with the help of AFS 410, we were able to make these changes. For more a detailed explanation, see the FAA/Industry Aeronautical Charting Forum agenda item history document on this particular AIM change:
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/acfipg/open/media/Hist%2008-01-279.pdf
With the publication of the AIM guidance, this matter will likely be closed at next month’s meeting. However, I remain troubled and concerned by the understanding of the OEI regulatory performance requirements by non-Part 121 operators, and particular by the Part 142 Training Organizations.
As you mentioned in your discussion, the performance requirements of TERPS (SIDs & ODPs) and the regulatory OEI performance requirements of Subpart I, Part 121 & Part 135 are two separate and independent subjects. Part 25 requires the aircraft manufacturers to provide the necessary performance data in the FAA-Approved AFM for the operator to show compliance with the operating rules contained in 91.605 and Subpart I, Part 121 & Part 135. TERPS performance requirements are not addressed by the operating rules; hence there is no requirement for the airframe OEM to provide all-engines operating performance data that can be used for TERPS performance compliance. However, there are efforts underway to address these requirements which are beyond the scope of the operating rules. See related ACF-IPG agenda item below:
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/acfipg/open/media/Hist%2098-01-197.pdf
On a related matter, additional guidance is provided in the 27 August 2009 AIM edition concerning compliance with climb gradients and altitude restrictions on ODPs and SIDs:
http://www.faa.gov/air_traffic/publications/ATPubs/AIM/CHG3AIM8-27-09.pdf
An ODP climb gradient is always established for obstacle or procedure design criteria purposes. A climb gradient or altitude restriction published on an ODP may not be canceled or amended lower by ATC. As noted above, ODP climb gradient requirements are based on normal, all-engines operating performance. SIDs are system enhancement procedures designed to facilitate air traffic clearance and traffic management. While assessed for obstacle clearance purposes per the TERPS criteria, climb gradients and altitude constraints may be published for air traffic separation/management purposes. These climb gradients and altitude constraints will be annotated “(ATC)” on future SIDs that are designed IAW FAA Order 8260.46D.
A climb gradient or altitude restriction published on a SID that is annotated (ATC) may be canceled by ATC at their discretion. On future SIDs, an (ATC) altitude restriction on the initial departure route will be accompanied by a second altitude restriction that is required for obstacle clearance purposes. This second altitude restriction is mandatory and cannot be amended by ATC.
The 27 August 2009 AIM change explains the differences between an ODP and SID climb gradient. An ODP climb gradient must always be complied with unless higher-than-standard takeoff minimums are published and the weather conditions at the time of takeoff meet or exceed those minimums. If the climb gradient is published without higher than standard takeoff minimums, then climb gradient is required, most likely for procedure design purposes. For an example, see the new RUUDY TWO RNAV SID for TEB runway 24. The initial climb gradient of 376 FPNM is required to support the turn to intercept the 260 track to WENTZ, which will occur at distance less than 2 NM from the departure end of runway 24. This climb gradient supports the need for the aircraft to be at least 400 ft above DER elevation before beginning a turn more than 15 degrees (see FAAO 8260.44).
91.175(f)(4) requires Part 121 and Part 135 operators departing IFR (not IMC, but rather on an IFR flight plan in either IMC or VMC conditions) to have a takeoff obstacle avoidance plan that meet the requirements of Subpart I, Part 121 or Part 135 as applicable:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=7e03cbb00617a7eecaaaf47605d76f4b&rgn=div8&view=text&node=14:2.0.1.3.10.2.6.42&idno=14
The Subpart I OEI takeoff obstacle clearance requirements are the same for both Part 121 and Part 135. They require the net takeoff flight path to clear obstacles by 35 ft and 200 ft laterally either side of flight track within the airport boundary, and by 300 ft laterally either side of flight track after passing the airport boundary. Subpart I also require the operator to account for effects of wind on the OEI flight track. AC 120-91, Airport Obstacle Analysis, provides guidance on operator compliance with this second requirement. The Obstacle Accountability Areas specified in the AC for straight-out and turning departures account for wind variations along the OEI lateral track without the need for specific wind accountability along the OEI flight track.
As noted in the AC, a Part 25 airplane’s OEI flight path is not linear, nor is it based on a single climb gradient value. Rather, the OEI flight profile is constructed of a series of flight path segments. The climb gradient charts provided for a Part 25 airplane provide for a value or “spot gradient” valid only at on particular point in the OEI climb. The 2nd segment climb gradient value may be the climb gradient attained at the point of landing gear retraction (e.g. Boeing & Dassault), 400 ft above the runway (e.g. Challenger 300), 1000 ft (e.g. Lear 45) or some other value. No two airframe OEM’s use the same method and will frequently vary with make and model.
The 2nd segment climb gradient value provides only a reference value for use with the takeoff flight path/obstacle charts. These flight path/obstacle charts are usually broken into close-in (1st & 2nd segment) and distant flight path charts. The flight path charts will usually contain some means of correction for effects of thrust lapse rate (thrust loss) through the OEI climb. Under no circumstances is it ever acceptable to use the 2nd segment climb gradient chart without reference to the flight path/obstacle charts. This is extrapolating data beyond the procedures found in the AFM, something the FAA very much frowns upon. Yet, I am well aware that this method is often taught at the Part 142 Learning Centers (who’s sign hangs over the door matters little).
Part 121 air carrier performance engineering departments or commercial performance engineering vendors (APG, Jeppesen OpsData, AeroData) use AFM takeoff flight path charts and performance data coupled with the “best available obstacle data“ (as required in AC 120-91) to verify OEI net takeoff flight path obstacle clearance that meet the requirements of Subpart I and in most cases meets the requirements of AC 120-91. If necessary to maximize takeoff weight, a special OEI procedure (EO procedure) may be provided. The EO procedures may follow a published ODP or SID, and operator’s special IFR departure procedure, e.g. the Eagle CO Cottonwood Special DP, or may be tailored EO procedure.
The OEI analysis does not always end at 1,500 ft. The OEI takeoff flight path per 25.111 ends at 1,500 ft, but AC 120-91 requires consideration beyond 1,500 ft into the enroute environment, a safe holding pattern, or return to the departure airport as appropriate. Also, Subpart I requires OEI obstacle clearance in the enroute phase of flight. The guiding principle of Subpart I is that, following an engine failure at or above V1, the airplane is assured obstacle clearance all the way to the destination an if necessary alternate airport (see 121.191/135.181).
TERPS criteria can form the basis of the OEI obstacle analysis, but as the AC 120-91 points out, the TERP climb gradient will be treated as a surface though which the airplane cannot penetrate from above. In other words, the OEI takeoff path must remain above the TERPS climb gradient surface or the OEI net takeoff flight path must remain 35 ft above the TERPS obstacle clearance surface (40:1 surface for a standard 200 FPNM CG). Further, there is a need to account for low, close-in obstacles which are not accounted for by the TERPS climb gradient. These are the obstacles found in the notes on ODPs or SIDS, or on the Jeppesen 10-4 OBST charts. These obstacles must be plotted on the AFM’s takeoff flight path/obstacle charts to verify obstacle clearance.
Since a TERPS climb gradient may be established for procedure criteria purposes as opposed for obstacle clearance purposes (see RUUDY TWO at TEB), using the TERPS climb gradient to meet Subpart I or 91.175(f)(4) takeoff obstacle clearance requirements may be unduly burdensome. The airport analysis method provides a much greater takeoff weight allowance.
There was much discussion on the FAA’s opinion on this subject. Rather say anything for the FAA, I thought you might like to hear it from the current manager of FAA’s AFS 410, Mr. Coby Johnson. The FAA produced an excellent training video hosted by Mr. Johnson for their inspector staff explaining AC 120-91, which may be viewed at this link:
http://videoontheweb.faa.gov/training/AFS-AC120-HS.asx
Please allow me to summarize a few key points:
- ODP & SID procedures assume all-engines operating performance.
- OEI contingency procedures are the responsibility of the operator.
- AC 120-91 Airport Obstacle Analysis provides operator guidance in developing these OEI contingency procedures.
- These OEI procedures and performance limits must be based on the data published in the AFM using the procedures published in the AFM. I have yet to see an AFM that instructs the user to apply a 2nd segment climb gradient to anything other than the flight path/obstacle charts.
- Independent use of a 2nd segment OEI climb gradient value derived from the AFM for TERPS compliance purposes in support of meeting Subpart I OEI net takeoff flight path obstacle clearance extrapolates data beyond the instructions provided in the AFM and is contrary to FAA policy.
- OEI takeoff obstacle clearance does not end at 1,500 ft. Rather it continues to the enroute structure where enroute obstacle clearance rule apply, or to safe location (holding pattern), or a return to the departure airport.
- All engines operating climb data is not provided in the AFM because Part 25 does not require such data since the operating rules in Parts 91, 121, and 135 do not address an all-engines operating obstacle clearance requirement.
- Aircraft departing under Instrument Flight Rules (on an IFR clearance – regardless of whether the weather conditions are VMC or IME) must have a takeoff obstacle avoidance plan that meets the requirements of Subpart I, Part 121 or Part 135 as applicable. The procedure need not follow or adhere to the TERPS ODP or SID procedure, although in most cases it would be beneficial if it did, especially in a non-radar environment.
I hope that this discussion was helpful in addressing this latest AIM change and recent FAR changes.
Best regards,
Rich Boll
