FMS-Calculated Climb Gradient Configuration/IAS

[dlrk]

I'm wondering what configuration and IAS/v-speed OCR climb gradient, req climb gradient, app/ldg climb gradient are based on. For example, if I'm wanting to make sure the airplane meets that performance, what do I need to do with regard to target speed and configuration?

[dlrk]

To be more specific, should I be maintaing v2 or vfto or some other speed to MSA to be clear of obstacles absent an ODP or SID?

[JJM]

13 hours ago, dlrk said:

To be more specific, should I be maintaing v2 or vfto or some other speed to MSA to be clear of obstacles absent an ODP or SID?

If there's no SID, there has to be an ODP. Someone correct me if I'm wrong.

[Rodeo]

Or a Diverse Departure Assessment, or the airport is not charted at all (e.g. some private airports) and the pilot is on his own.

Still doesn't answer the OP's question about which airspeed and/or climb configuration corresponds to the FMS-calculated climb performance though…

[richjb]

On 4/9/2022 at 7:31 AM, JJM said:

If there's no SID, there has to be an ODP. Someone correct me if I'm wrong.

Not necessarily...  I'm speaking for the US now.   Every airport with an instrument approach is assessed for an IFR departure.  If you want to know the details, you can read about it in the FAA's Instrument Procedures Handbook: Instrument Procedures Handbook (IPH) | Federal Aviation Administration (faa.gov).   If there are no obstacles or terrain affecting an IFR departure from the runway, the pilot is free to turn in any direction after climbing to 400' above the runway elevation and proceed on their way.  It's important to note that this "diverse" departure assessment is valid only out to 25 miles in nonmountainous areas and 46 miles in mountainous areas.  If obstacles or terrain effect the IFR departure, then takeoff minimums and/or an obstacle departure procedure (ODP) is published for that runway.  The ODP can be nothing more than listing of low, close in obstacles that must be seen or otherwise avoided on takeoff.  These are not a factor in turbojet takeoffs as long as all engines are operating.  The takeoff performance requirements mean that you are well above most of these obstacles by the time you cross the end of the runway.  The ODP can consist of a higher than standard climb gradient that will clear obstacles or higher than standard takeoff minimums that allow the pilot to see an avoid the obstacles.  The ODP can also turn restrictions, such climb on a heading to an altitude before turning in a specific direction.  Finally, the ODP can consist of specific routing to avoid obstacles, to include a climb in holding until reaching a safety altitude.  

A SID is nothing more than an abbreviated ATC clearance in charted form. The route is evaluated for obstacle clearance, and if assigned by ATC you fly it instead of the ODP.  If no SID is assigned, but takeoff minimums and/or an ODP is published for the runway, and you are an FAA certificated operator (i.e., airline or charter operator), then you're required to fly the ODP. 

If there is no SID or ODP, then you are free to turn in any direction and proceed on your way to your first en route fix.  However, again you are responsible for terrain and obstacle clearance once you're more than 25/46 miles from the airport. 

Hope this helps,

Rich Boll

[richjb]

On 4/8/2022 at 2:15 PM, dlrk said:

I'm wondering what configuration and IAS/v-speed OCR climb gradient, req climb gradient, app/ldg climb gradient are based on. For example, if I'm wanting to make sure the airplane meets that performance, what do I need to do with regard to target speed and configuration?

So, I'm going to preface this by saying that this is how it works in the Challenger 300/350, which I fly....  I don't fly the 650 and have not yet looked that close at it, but I have no reason to believe Bombardier and Collins would approach this any differently between airplanes. 

TAKEOFF REF page 3/4 presents several options that you can enter (see attachment).  One is OBST HT and OBST DIST.  LEVEL-OFF HT is also included.  You enter the obstacle height and distance from "reference zero", which is the end of the takeoff distance to 35 feet (dry) or 15 feet (wet). Because they measure the obstacle's distance from reference zero, unused runway must be added to the obstacle's distance from the end of the runway.   This also is a clue as to what data Collins and Bombardier are using with the calculation. 

This "Obstacle Clearance Calculator" that Bombardier & Collins are using references the AFM Performance Section's Flight Path charts, both close-in and distant.  These charts have been digitized and stored in the FMS's performance module. The Obstacle Clearance Calculator is using the obstacle distance from reference zero and the obstacle's height above the runway elevation to determine the required Reference Climb Gradient necessary to clear the obstacle.  The reference climb gradient is equal to the 2nd segment climb gradient achieved after the landing gear has retracted, but the flaps are still in the takeoff configuration, and the thrust is at the takeoff thrust setting.  The airspeed is V2 speed.  Further, all of this data is based on an engine failure at VEF or the presumed engine failure speed, which is typically 1 second prior to V1 speed with the pilot electing to continue the takeoff. 

The calculator will also calculate the level-off height necessary to clear the obstacle in this second segment climb, after which the aircraft can level, accelerate, and clean up to VFTO speed.  That is of course assuming that this is the most critical obstacle for the takeoff.  The FMS defaults to a 1000' level-off height but can extend to a higher level off height as needed.  It can also be set to minimum height of 400'.  

The AFM's Flight Path graphic illustrates the profile that is flown.  The FMS Takeoff Ref Page 3/4 is calculating the required climb gradient flight path during the first segment climb and the second segment climb up to the calculated Gross Level-Off Height.  The required Reference Climb Gradient will ensure that the Net Takeoff Flight Path clears the obstacle.  The actual, or "Gross" flight path will clear the obstacle by much more, approximately 0.8% of the distance traveled from reference zero.  

The Obstacle Clearance Limit weight is calculated based on the data you entered. 

The system can work backwards, too.  You can enter a required climb gradient in the REQ CLB GRAD field and have it calculated the limit weight.  The problem is that we don't know that value. It's not the SID or ODP climb gradient since 1) those are based on all-engines operating, normal takeoff performance, and 2) the reference climb gradient data in the FMS is based on the limited scope of the two flight path charts, close-in and distant.  On the distant chart, once you get beyond that vertical line on the right of the chart, you're exceeding the 5 minute time limit for takeoff thrust. 

If you want to know how to comply with a SID or ODP climb gradient, I refer you to this FAA Information for Operators (InFO) bulletin: InFO 18014: Compliance with Title 14 of the Code of Federal Regulations (14 CFR) Part 97 Instrument Flight Rules (IFR) Departure Procedure & Missed Approach Climb Gradient Requirements (faa.gov)

The more I look at the CL650's FMS guide and the AFM, the more that I am fairly certain that it works the same way as in our CL300/350.

We do not use this FMS feature.  Rather we along with most other operators are now using some type airport runway analysis.  In the flight simulator world, these are the type of takeoff performance reports generated by the TOPCAT system: FlightSimSoft.com.  Unfortunately, we're not likely to see the CL350 added to the list of airplanes supported. 

I hope this helps!

Rich Boll

Edited April 16, 2022 by richjb
Grammatical Corrections :-), and some clarifications

[richjb]

On 4/8/2022 at 2:15 PM, dlrk said:

I'm wondering what configuration and IAS/v-speed OCR climb gradient, req climb gradient, app/ldg climb gradient are based on. For example, if I'm wanting to make sure the airplane meets that performance, what do I need to do with regard to target speed and configuration?

 I neglected to answer your questions on approach climb and landing climb gradients, and what they are based on here.  What we have in the FMS is a good hearted attempt to provide information to the pilot that is really relevant to performance.  I'll explain in a moment.

Approach climb is certification performance requirement. It is the climb gradient available with the aircraft in the approach configuration (flaps 20 for the CL65), landing gear retracted, with the thrust set to go-around thrust, a climb speed not more than 1.4 VSR (reference stall speed), and based on the published AFM procedures for a OEI go around. The minimum climb gradient is 2.1%. 

Landing climb is also a certification performance requirement.  It is the climb gradient available in the landing configuration, landing gear extended, at VREF speed, and with thrust available 8 seconds after go-around thrust is selected (which may be less than the actual go-around thrust as the engine spools up). The minimum climb gradient is 3.2%.  

These two climb gradients are "certification requirements" and established as part of the maximum allowable landing weight for the aircraft in the AFM Limitations Section.  For the CL650, they are published in the AFM Performance Section, but incorporated into the Limitations section by reference.  The operating regulations for flying the airplane require that airplane's takeoff weight, less fuel and oil consumed in flight, allow the airplane to arrive at the destination airport and alternate airport (if applicable) at weight that is less than the maximum allowable landing where the approach climb and landing climb requirements are met.  

The AFM has an Approach and Landing Climb weight limit chart.  That chart is incorporated into the FMS performance module.  When you complete the APPROACH performance page, if you are overweight, for the information that you entered for landing, the FMS will tell you:

 

MLW
Maximum Landing Weight (MLW) is determined by the most restrictive
selection among the following: structural limit weight, climb performance
limit weight, and runway length limit weight. If MLW is unable to be
computed, the MLW data field is blank.
When landing weight is greater than maximum landing weight, the MLW
shows in yellow. In addition, a CDU message CHECK APPROACH
PERF shows under these conditions.
MLW values are synchronized on the APPROACH REF pages

All of the one-engine-inoperative (OEI) takeoff, OEI approach climb, and all-engines-operating (AEO) Landing climb gradient requirement are not true climb performance requirements.  They are statements of excess energy available to the aircraft in the stated configuration, at the applicable speed, and with the available thrust.  That excess energy can be used to climb the aircraft or accelerate it, as appropriate.  Further, the climb gradient values are "spot gradients" that a valid only at particular point in takeoff or landing phase.  For example, the 2nd segment climb gradient data and climb gradient value determined is only valid at the exact instant that the landing gear is fully retracted after takeoff at V2 speed, with the takeoff flaps, and with takeoff thrust with one engine inoperative.  Once you leave that "spot", the climb gradient decrease.  For 2nd segment, or reference climb gradient as Bombardier calls it, the decrement in climb gradient that occurs as the aircraft climbs is built into the two flight path charts that I posted earlier, and is why the takeoff flight path charts are used to determine obstacle clearance rather than computing a "rise over run" climb gradient requirement, e.g., 300' obstacle, 1 NM away, minimum climb gradient 300 ft/NM or 4.9%. That doesn't work out that way when you use the flight path charts for the reference climb gradient required for obstacle clearance.  Using the flight path chart, the reference climb gradient required to clear a 300' obstacle 1 NM (6076 feet) from the runway end (assuming takeoff distance required is equal to the runway length) is 5.3% (see example).

 

As long as your Maximum Landing Weight is not showing overweight in the FMS for the conditions you entered, follow the AFM go around procedures, and you will meet the performance requirements.  For OEI go around, climb a speed not less than VAPP and for all engines missed approach, a speed not less than VREF.  However, at most landing weights, you'll be well below these limits and the AFM procedure will likely result in speeds being attained above these limits at the recommended go-around pitch attitude.  If that's the case, take the extra speed. 

Rich Boll

 

[richjb]

On 4/8/2022 at 5:57 PM, dlrk said:

To be more specific, should I be maintaing v2 or vfto or some other speed to MSA to be clear of obstacles absent an ODP or SID?

Typically, the normal takeoff procedure will result in you climbing well above any climb gradient requirement on a SID or ODP. If there is no SID or ODP, then a 200 ft/NM climb gradient is required.  For takeoff the minimum takeoff speed, all engines operating is V2+10.  That provides you sufficient stall margin with 30 degree bank.  

I can get into a whole bunch calculations, etc. concern how to compute a percent climb gradient and rate of climb when you have a SID or ODP climb gradient in feet/NM.  The Collins Proline 21 has a neat feature that makes this easy.  Use the Flight Path Vector. 

If you have a SID with a 400 ft/NM climb gradient, after takeoff make sure that the Flight Path Vector remains 4.0 degrees or more above the horizon.  The climb gradient in feet/NM roughly equals the flight path angle in degrees - See the US Government Terminal Procedures Publication Legend below. 

Turbojets are generally capable of meeting a 500 ft/NM climb gradient on a SID or ODP without much further consideration up to 1500' above the airport elevation simply on the basis of the aircraft's OEI takeoff climb requirements.  Again, it is not required nor expected that you meet the SID or ODP climb gradient with OEI. 

Initially, rotate to the TO command bar pitch attitude.  That's to prevent on over-rotation accident.  After the aircraft is accelerating, then use FLC and 200 Kts to continue the climb.  See FCOM 1 takeoff procedure below.

Rich  

[dlrk]

Thanks for all this great information! I agree that FlightSimSoft will probably never update TOPCAT, but perhaps Apsoft's Toolbox will add a CL650 down the line.

I do have a one question. I thought the climb gradient on an ODP would be that required to avoid terrain. So, why isn't it required to meet that with OEI?

[richjb]

12 hours ago, dlrk said:

Thanks for all this great information! I agree that FlightSimSoft will probably never update TOPCAT, but perhaps Apsoft's Toolbox will add a CL650 down the line.

I do have a one question. I thought the climb gradient on an ODP would be that required to avoid terrain. So, why isn't it required to meet that with OEI?

 

Yes, an ODP is an IFR procedure used to avoid terrain.  However, it's the context that is important to understand. 

You have asked a question that has caused and continues to cause much confusion among pilots and their training providers.  It is one reason why the FAA produced the set of training videos on transport airplane performance, which I have linked in another post.

In the US, the certification & operating rules for transport category airplanes, which apply to the CL650, date back to June 1941.  That's before Pearl Harbor!  While the certification rules have been updated to reflect changes in airplane technology, namely the introduction of turbojet airplanes in the 1950s, the operating rules remain largely unchanged.  For these airplanes, a basic concept applies.  When operated in commercial service (in the US, under part 121 or part 135 - the later applying to the CL650), The concept is that the airplane can experience an engine failure prior to V1 speed and stop on the remaining runway available, or experience an engine failure after V1, continue the takeoff, and the entire flight to the destination airport and alternate airport, if required, without hitting an obstacle.  If you read the operating rules, this concept is clear.  And this basically how obstacle clearance was addressed even for a normal IFR takeoff all-engines operating since it was always considered, and still is, that the obstacle clearance following an engine failure is more critical than it would be for a normal takeoff. 

The FAA did not begin to address obstacle clearance on an IFR takeoff until a major re-write to their instrument procedure criteria in 1967, which gave us the US Standard for Terminal Procedures, or US TERPS.  Even with this change, it took until the mid-70s until the instrument departure procedure criteria resembles what it looks like to today.   At that time, they looked at the possibility of including a OEI requirement to the criteria.  After much input from the industry, including the airlines which had been developing OEI obstacle avoidance procedures since the 40's and objected to this addition, the FAA decided that the IFR takeoff and departure, and all of TERPS,  will only address only normal aircraft operation.   The Title Page to the US TERPS Order 8260.3E states the following:

This order prescribes standardized methods for designing and evaluating instrument flight procedures (IFPs) in the United States and its territories. It is to be used by all personnel responsible for the preparation, approval, and promulgation of IFPs. These criteria are predicated on normal aircraft operations and performance.

In paragraph 2-1-4 e, the Order further states:

e.  Extraordinary circumstances, such as a mechanical or electrical malfunction, may prevent an aircraft from achieving the 200 ft/NM minimum climb gradient assumed by TERPS. In these cases, adequate obstacle clearance may not be provided by published IFPs. Operational procedures contained outside TERPS guidelines are required to cope with these abnormal scenarios

Those "operational procedures contained outside of TERPS guidelines" are the operating rules with respect to OEI takeoff and en route obstacle clearance.  In short, the criteria used to develop an ODP or a SID assumes a normally operating aircraft, with all engines operating.  When the engine quits, the ODP or SID no longer applicable because the criteria doesn't use the same assumptions that the airplane certification criteria uses to construct the OEI takeoff flight path shown in the excerpts above in the AFM.  For example, an ODP or SID's climb gradient assumes a linear climb to the climb gradient termination altitude.  Actually, it's a surface that cannot be penetrated from above.  The OEI takeoff flight path is not linear, it's a segmented path, and obstacle clearance needs to be assessed at each point along that path.  Also, there are obstacles that are not considered by the ODP or SID climb gradient that could still be factor following an engine failure.  These are known as "low, close-in obstacles".  

Low, close-in obstacles are obstacles that lie right off the end of the runway.  They require an IFR climb gradient greater than the standard 200 ft/NM, but that climb gradient would only be required to a height of less than 200 feet above the runway.  Rather than publishing the climb gradient, they publish the obstacles and their location relative to the runway end so that the pilot can take whatever action they deem appropriate to miss them. 

One of my favorite examples of how low, close-in obstacles can affect takeoff performance is Hutchison KS, runway 13 at 40C (104F).  Yes, living in Wichita which is just down the road, 40C in the summertime is not unusual. if you run takeoff data for just the runway length and the standard 200 ft/NM or 3.3% climb gradient off runway 13, you find a weight that is greater than the weight limit shown on a takeoff runway analysis that accounts for these obstacles.  Therein lies the problem for commercial operators who relay solely on IFR ODP or SID climb gradient.  They takeoff at weight in excess of that required by the CFRs (e.g., 14 CFR 315.379 d ii) and if the engine fails at V1, they could find themselves tangling with the obstacles while the landing gear is retracting.  Private operators who not governed by these OEI takeoff obstacle clearance rules often abide by them voluntarily. 

The US Aeronautical Information Manual has the following statements in Section 5-2-9 concerning ODP and SIDs:

2. ODPs, SIDs, and DVAs assume normal aircraft performance, and that all engines are operating. Development of contingency procedures, required to cover the case of an engine failure or other emergency in flight that may occur after liftoff, is the responsibility of the operator. (More detailed information on this subject is available in Advisory Circular AC 120−91, Airport Obstacle Analysis, and in the “Departure Procedures” section of chapter 2 in the Instrument Procedures Handbook, FAA−H−8083−16.)

NOTE−
Compliance with 14 CFR Part 121 or 135 one−engine−inoperative (OEI) departure performance requirements, or similar ICAO/State rules, cannot be assured by the sole use of “low, close−in” obstacle data as published in the TPP. Operators should refer to precise data sources (for example, GIS database, etc.) specifically intended for OEI departure planning for those operations.

(d) Consider the effect of degraded climb performance and the actions to take in the event of an engine loss during the departure. Pilots should notify ATC as soon as possible of reduced climb capability in that circumstance.
NOTE−
Guidance concerning contingency procedures that address an engine failure on takeoff after V1 speed on a large or turbine−powered transport category airplane may be found in AC 120−91, Airport Obstacle Analysis.

For those folks flying in Europe and throughout the world, ICAO PANS-OPS contains similar statements.  Obstacle clearance following an engine failure on takeoff is the responsibly of the operator and is outside the scope of US TERPS and ICAO-PANS OPS. 

How these OEI takeoff obstacle avoidance procedures are developed is described in FAA AC 120-91A: AC 120-91A - Airport Obstacle Analysis – Document Information (faa.gov)

I hope this helps explain the difference.

Rich Boll

 

 

 

[dlrk]

Okay, so if I understand correctly, while the ODP does not account for OEI terrain clearance, the FMS calculator does (provided the necessary data is inserted)?

[richjb]

6 hours ago, dlrk said:

Okay, so if I understand correctly, while the ODP does not account for OEI terrain clearance, the FMS calculator does (provided the necessary data is inserted)?

The ODP or SID is not intended for OEI terrain and obstacle clearance since the criteria does not consider both the actual takeoff flight path of the aircraft following an engine failure nor does the climb gradient on an ODP or SD account for all obstacles that much cleared to meet the operating rules.  An ODP or SID provides obstacle clearance with all-engines-operating because that's what its criteria assumes.  Make sense? 

With the proper obstacle information concerning obstacle height above the runway and distance from reference zero, the FMS calculator can be used to calculate OEI takeoff obstacle clearance in accordance with the operating rules.  Here's an important caveat.  You need the relevant terrain & obstacle data and collecting that data no small feat.  There are multiple data sources that need to be consulted, which is why the airlines have performance engineering departments dedicated to that task.  For the business aviation community, we have contract providers such Aircraft Performance Group, ASAP Inc., Aerodata (now owned by Garmin), and Jeppesen OpsData through ForeFlight that provide this type of engineering support, and provide takeoff performance/obstacle clearance data in the form of airport runway analysis.  

As real CL300/350 pilot, I would never attempt to gather the obstacle data from the various sources and use the FMS calculator, in the same way that I would never attempt collect obstacle data and build my own instrument approach procedure to a runway.  The FAA is expert in building instrument flight procedures and these performance engineer providers are the experts in airplane performance and engine failure escape procedures. 

I was never really quite sure why the FMS manufacturers (OEMs) put the obstacle clearance routine in their FMS.  It's not unique to Collins. It's in the Garmin 5000 as well.  If you had one known obstacle that you wanted to clear, one that as not accounted for in your contractor-provided analysis, for example a temporary obstacle, then yes, I could see a use for it.  Although in 18 years of flying Collins FMS equipped aircraft, I have never used it other than for experimentation and familiarization. 

Rich 

 

Edited April 18, 2022 by richjb
typo correction...