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More than you ever wanted to know…

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1 More than you ever wanted to know…
KC-135R/T Climb Gradient More than you ever wanted to know… Capt Don Kennedy 55ARS/STM Altus AFB, OK

2 Overview Purpose Motivation Review of Regulatory Requirements
Explanation of KC-135 Climb Gradient Flap/Profile Selection Effect of Speed Deviation on Climbout Flightpath FSAS Techniques Approach & Go-Around Future Developments

3 Purpose To arm you with the knowledge and tools to make sound takeoff planning decisions No imposition of my techniques or philosophy… I want to help you develop your own However, give me the opportunity to sell you on these ideas with facts and sound rationale Above all else… I WANT YOU TO THINK

4 … it is designed to potentially save your life
Motivation This discussion is not merely theoretical, designed solely to impress your pilot friends at parties… … it is designed to potentially save your life

5 Regulatory Requirements
AFI V3, General Flight Rules

6 Regulatory Requirements
AFI 11-2KC-135V3, C/KC-135 Operations Procedures

7 Regulatory Requirements
AFMAN V1, Instrument Flight Procedures

8 Regulatory Requirements
Aeronautical Information Manual, 5-2-6 “1. Unless specified otherwise, required obstacle clearance for all departures, including diverse, is based on the pilot crossing the departure end of the runway at least 35 feet above the departure end of runway elevation, climbing to 400 feet above the departure end of runway elevation before making the initial turn, and maintaining a minimum climb gradient of 200 feet per nautical mile (FPNM), unless required to level off by a crossing restriction, until the minimum IFR altitude.”

9 Regulatory Requirements
T.O. 1C-135(K)R-1-1, Flight Manual Performance Data, 1A3-20A, Climbout Procedure, Note 2 “The minimum planned clearance over an obstacle is 1.0% of the obstacle distance from the end of the runway.” 6076(.01)=60.76 or 61 feet/nm If your standard OIS was 152 feet/nm, then = 213 Therefore, Boeing says that 213 feet/nm, or 3.5% is your minimum 3-engine climb gradient

10 Regulatory Requirements
So what clearance requirement am I supposed to satisfy? Boeing’s 61 feet? AIM’s 48 feet? AF’s 0 feet? Kennedy’s Opinion (Peacetime Criteria) 61’: Will satisfy if able 0’: Unsafe 48’: Appropriate 213 (61) 200 (48) 152 (0) Obstacle Clearance Plains – Feet/NM (Height Above)

11 KC-135 Climb Gradient Climb Gradient… (Performance Data, 1A3-4)
“Climb gradient is the flightpath climb angle expressed in percent, and equates to feet climbed per 100 feet of horizontal distance traveled.” “Climb gradient is a direct measure of the takeoff acceleration and climb capability of the airplane.” “Climb gradient is used as a normalizing parameter to simplify Part 3 performance charts.” So which is it? Height gain per distance traveled? (true climb gradient) Measure of performance? Normalizing parameter?

12 KC-135 Climb Gradient The answer is, “YES!”
All three statements are true Height gain per distance traveled Measure of performance Normalizing parameter Boeing uses true climb gradient as a measure of aircraft performance (thrust excess) to normalize your takeoff data Normalizing… meaning climb gradient is the parameter that accounts for temp, PA, gross weight, thrust setting, etc. that characterize your takeoff

13 KC-135 Climb Gradient “Since climb gradient equates the feet climbed per 100 feet horizontal distance traveled, the climb gradient can be used to find the minimum height at a given obstacle distance. This method is different than that used for obstacle clearance in Part 3, where additional charts are needed to determine obstacle clearance. The need for additional charts is a result of the climbout flightpath angle. This is not a constant value, and thus one climb gradient cannot be assigned to it.” - Boeing response to field visit questions, Sep 04

14 KC-135 Climb Gradient Although “True Climb Gradient” is perfectly linear, airplanes do not fly straight-line flight paths during the takeoff phase Changing pitch Acceleration Ground effect Gear or flap retraction Pilot technique Thrust lapse rate Changing environmental conditions with altitude

15 KC-135 Climb Gradient Okay, fine… all this theory is nice, but what I really care about is HOW DO I USE IT??? You are really interested in Climbout Flightpath “How high am I at a certain distance if I am trying to clear an obstacle or maintain a min feet/nm?” DO NOT use the Climb Gradient the FSAS gives you (i.e. 8%) as a direct measure of your ability to meet your IFR climb criteria (feet/NM) or ability to clear an obstacle Climb Gradient ≠ Climbout Flightpath Remember… your Climbout Flightpath is highly non-linear, which is why we have flight-test data for height above unstick

16 KC-135 Climb Gradient Proof that Climb Gradient ≠ Climb Gradient
Conditions: 205,000 lbs 27% C.G. Temp: 25°C PA: 1000 Winds: 030°/10 If the jet really flew an “8% Climb Gradient,” at 1 mile from the end of the runway it should be at least 486 feet high (6076 x .08 = 486) Runway Length: 11,200’ 20 Flap Accel Profile Climb Gradient = 8% UOD: 11,149’ OD: 6076’ (1 mile)


18 FSAS Calculator computed a DHT of 328’
KC-135 Climb Gradient 330’ FSAS Calculator computed a DHT of 328’

19 KC-135 Climb Gradient If the jet really flew an “8% Climb Gradient,” at 1 mile from the end of the runway it should be at least 486’ feet high (6076 x .08 = 486) Charts: 330’ FSAS: 328’ 330 ft/nm = 4.9% “True Climb Gradient” Therefore, you can’t correlate the charted climb gradient to a desired height vs. distance gained for the purpose of obstacle clearance!

20 Flap/Profile Selection
If the charted or FSAS-calculated Climb Gradient is a measure of thrust excess, then the pilot can use it in three ways: Climb Accelerate Both climb and accelerate (tradeoff) Selection of takeoff flap setting and profile then, is just a decision about how to use your Climb Gradient “How do I need/want to use my performance today?”

21 Flap/Profile Selection
Performance Manual’s preference order 20 ACCL, 20 MAX, 30 MAX, 30 ACCL Pilot’s prerogative to choose What are some caveats to this precedence? Obstacle clearance Runway available Ground min control speed You must understand why you have selected a particular flap setting and profile “We always do 20 ACCL at my base” is unacceptable

22 Flap/Profile Selection
Age old question… “What is the best flap setting and profile?” The answer: “Well, that depends.” Factors to consider when selecting a profile Runway available Runway end crossing requirements Gross weight Min IFR/SID climb gradient compliance Obstacle clearance (and how far away the obstacle is) Reduced thrust N1 setting Controllability/stall margin Operational/Training Requirements

23 Flap/Profile Selection
Benefits of the ACCL mode takeoff Increased stall margin Quicker acceleration through region of reverse command (quicker flap retraction) Improved controllability Improved distant obstacle clearance at lighter gross weights Improved windshear penetration

24 Flap/Profile Selection
Pitfalls of the ACCL mode takeoff Relatively shallow initial climbout flightpaths Close-in obstacle clearance is problematic No way to validate FD109 pitch commands More procedurally difficult during a critical phase of flight with engine loss and/or FD109 failure These qualities present an obvious safety concern, which is why some advocate elimination of the ACCL profile. The arena in which the ACCL mode shines (heavy/performance limited) is precisely where its pitfalls render it illegal, unsafe or impractical, due to inadequate obstacle clearance and an inability to validate the FD109-commanded flightpath.

25 Flap/Profile Selection
“So which flap setting is better for obstacle clearance, 20 or 30?” Answer: “It depends!” 30 for close-in obstacles (less than ~3 miles) 20 for distant obstacles (greater than ~3 miles)

26 Flap/Profile Selection
Gross Weight: 288K lbs RA: 9700’ Temp: 20C PA: 1000’ N1: TRT

27 Effect of Speed Deviation
Your charted climbout flightpath is predicated upon you correctly setting the computed takeoff N1 and flying within the allowable speed deviation 20 Flap: 8 knots 30 Flap: 3 knots Remember… for a fixed thrust setting, acceleration comes at a price (less climb) Exceeding your speed deviation invalidates your takeoff solution

28 Gross: 288K lbs RA: 11800’ T: 30C PA: 1000’ MCT: 89.4%N1 200'/NM No tech order data for exceeding speed dev… this example, using increased rotation is anecdotal evidence to illustrate what happens when you exceed your speed deviation On Speed +10kts

29 FSAS Techniques Alright Kennedy, I’m sick of theory… just show me how not to hit stuff if I lose an engine on takeoff! USE THE FSAS—it is easier! The Part 3 charts use Unstick to Obstacle Distance (UOD) whereas the FSAS deals in Obstacle Distance (OD) from end of runway This makes the FSAS easier, because IFR climb gradients and published obstacles are given from the end of the runway

30 How to Use the FSAS (Reference cheat sheet at end of slides)

31 How to Use the FSAS

32 How to Use the FSAS The PC version does the same thing

33 How to Use the FSAS Use the FSAS iteratively to check your actual climbout flightpath Runway end crossing height – “[OD/H] of [1/1]” Check at 1nm (6076 feet) or and at multiple points along your flightpath until you are satisfied Your FSAS uses the 1% of obstacle distance rule as an acceptable crossing height “T/O NOGO” will be displayed if DHT is less than 61’/nm Consider using the “1 foot obstacle technique” For example: for [OD/H] Enter [6076/1] Will yield an height above departure end of runway elevation

34 Future Developments SID Compliance Charts
Will tell you what FSAS Climb Gradient you need to enter to achieve a given published climb requirement Will help eliminate the highly iterative process of using the FSAS DHT function or Part 3 charts Easier to determine runway end crossing height compliance Will drive gross weight reductions in some cases to meet min IFR takeoff criteria


36 Approach & Go-Around Misconceptions abound…
Required Climb Gradient (real engine out) Missed approach… V3 & V1: 200’/nm or 3.3% But what about 11-2KC-135V ? (practice) touchdown speed, 30° Flaps, gear, …” If I put [1/4] for engines out in the FSAS, won’t I get a symmetric 2E climb gradient? NO! Is that a 3.3% off the FSAS or is that 3.3% “true climb gradient?” 3.3%

37 Approach & Go-Around The answer is… YES!
In this case, they are the same! Can I use the FSAS as direct measurement for missed approach or go-around? YES! Yes… and Boeing explains it best: “The charts in Part 9 were built using a different method, one using a “true” minimum climb gradient, which can be conservatively used for obstacle clearance planning.” So if my FSAS reads 3.3 or higher, am I good? In reality, you have something greater, as the 3E FSAS climb gradient is 50 Flap, not 30 Flap

38 Practice 3-Engine Work The issue typically isn’t meeting a 3.3% climb gradient for 3E go-around performance at transition weights The question you’re really concerned about is… When do I need to use the asymmetric throttle? If your 2E (same side) climb gradient is 3.3 or above, then your actual climb gradient (with 2 engines at idle and the other 2 at symmetric N1) will be something above 3.3 If you don’t get 3.3 off the FSAS for 2E, you’ll have to run the chart … Fig 1A9-8

39 The Bottom Line Peacetime: make every effort to meet 200’/nm (or higher, if published) 3-engine Don’t use the Climb Gradient from the FSAS as a direct measure of climb performance—use the DHT iteratively to determine if you meet or exceed the required climbout flightpath Fly ACCL takeoffs if you wish—just understand that when conditions exist for its benefits to shine, such benefit is negated by its limitations Don’t blow off your speed deviation when flying a MAX mode profile—it is what validates your obstacle clearance


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