KC-135R/T Climb Gradient

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KC-135R/T Climb Gradient

• More than you ever wanted to know

Capt Don Kennedy 55ARS/STM Altus AFB, OK
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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

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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

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Motivation

• This discussion is not merely theoretical,
  designed solely to impress your pilot friends at
  parties

it is designed to potentially save your life
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Regulatory Requirements
AFI 11-202V3, General Flight Rules
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Regulatory Requirements
AFI 11-2KC-135V3, C/KC-135 Operations Procedures
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Regulatory Requirements
AFMAN 11-217V1, Instrument Flight Procedures
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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.

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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 152
  61 213
• Therefore, Boeing says that 213 feet/nm, or 3.5
  is your minimum 3-engine climb gradient

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Regulatory Requirements

• So what clearance requirement am I supposed to
  satisfy?
• Boeings 61 feet?
• AIMs 48 feet?
• AFs 0 feet?
• Kennedys Opinion (Peacetime Criteria)
• 61 Will satisfy if able
• 0 Unsafe
• 48 Appropriate

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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?

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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

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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

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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

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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

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KC-135 Climb Gradient

• Proof that Climb Gradient ? Climb Gradient
• Conditions
• 205,000 lbs
• 27 C.G.
• Temp 25C
• 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)

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KC-135 Climb Gradient
CHART USED IS SPECIFIC FOR EACH T/O PROFILE AND
FOR CLOSE IN VS. DISTANCE OBSTACLES!
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KC-135 Climb Gradient
FSAS Calculator computed a DHT of 328
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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 cant correlate the charted climb
  gradient to a desired height vs. distance gained
  for the purpose of obstacle clearance!

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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?

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Flap/Profile Selection

• Performance Manuals preference order
• 20 ACCL, 20 MAX, 30 MAX, 30 ACCL
• Pilots 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

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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

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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

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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.
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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)

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

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Gross 288K lbs RA 11800 T 30C PA 1000 MCT
89.4N1
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
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FSAS Techniques

• Alright Kennedy, Im sick of theory just show me
  how not to hit stuff if I lose an engine on
  takeoff!
• USE THE FSASit 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

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How to Use the FSAS
(Reference cheat sheet at end of slides)
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How to Use the FSAS
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How to Use the FSAS
The PC version does the same thing
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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

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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

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TRAINING/REFERENCE ONLY
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Approach Go-Around

• Misconceptions abound
• Required Climb Gradient (real engine out)
• Missed approach 11-202V3 11-217V1 200/nm or
  3.3
• But what about 11-2KC-135V3 9.6.2? (practice)
• 2.8 _at_ touchdown speed, 30 Flaps, gear,
• If I put 1/4 for engines out in the FSAS, wont
  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
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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

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Practice 3-Engine Work

• The issue typically isnt meeting a 3.3 climb
  gradient for 3E go-around performance at
  transition weights
• The question youre 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 dont get 3.3 off the FSAS for 2E, youll
  have to run the chart Fig 1A9-8

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The Bottom Line

• Peacetime make every effort to meet 200/nm (or
  higher, if published) 3-engine
• Dont use the Climb Gradient from the FSAS as a
  direct measure of climb performanceuse the DHT
  iteratively to determine if you meet or exceed
  the required climbout flightpath
• Fly ACCL takeoffs if you wishjust understand
  that when conditions exist for its benefits to
  shine, such benefit is negated by its limitations
• Dont blow off your speed deviation when flying a
  MAX mode profileit is what validates your
  obstacle clearance

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