kc-135r/t climb gradient

KC-135R/T Climb Gradient

More than you ever wanted to know…

Capt Don Kennedy55ARS/STM

Altus AFB, OK

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

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

Motivation

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

… it is designed to potentially save your life

Regulatory Requirements

AFI 11-202V3, General Flight Rules


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


AFMAN 11-217V1, Instrument Flight Procedures


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.”


• 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


• 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

0

200

400

600

800

1000

1200

1400

1600

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Distance from End of Runway (ft)

Hei

ght G

ain

(ft)

Distance (OD) 152'/nm (2.5%)



213 (61)

200 (48)

152 (0)

Obstacle Clearance Plains – Feet/NM (Height Above)

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?


• 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


“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


• 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


• 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

• 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)



CHART USED IS SPECIFIC FOR EACH T/O PROFILE AND FOR CLOSE IN VS.

DISTANCE OBSTACLES!

CHART USED IS SPECIFIC FOR EACH T/O PROFILE AND FOR CLOSE IN VS.

DISTANCE OBSTACLES!


330’

FSAS Calculator computed a DHT of 328’


• 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!

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?”


• 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


• 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


• 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


• 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.


• “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)

Flap/Profile SelectionClimbout Flightpath Comparison - High Gross Weight

0

500

1000

1500

2000

2500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


He

igh

t G

ain

(ft

)




Distance (OD) 20ACCL

Distance (OD) 30MAX

Distance (OD) 20MAX

Gross Weight: 288K lbs

RA: 9700’

Temp: 20C

PA: 1000’

N1: TRT

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

Effect of Increased Rotation Speed of 10 Knots

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14


Hei

gh

t Gai

n (f

t)

+10kts

On Speed

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

Gross: 288K lbs

RA: 11800’

T: 30C

PA: 1000’

MCT: 89.4%N1

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

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

How to Use the FSAS

How to Use the FSAS

The PC version does the same thing

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

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

TRAINING/REFERENCE ONLY

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% @ 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%

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

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

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

KC-135R/T Obstacle Clearance Verification The term “Climb Gradient” has two distinct means of application, that if interchanged, could kill you. For example, if a SID requires a 400 feet/nm climb (6.6% climb gradient) for obstacle clearance and your FSAS data says you have a 8.0% climb gradient, you may or may not be able to clear that obstacle. These two “climb gradients” are related, but only indirectly. Therefore, do not directly equate one with the other! Climb Gradient (1): (FAA, TERPS, AFI11-202V3, etc.) The flightpath climb angle, expressed in percent, and equates to feet climbed per 100 feet of horizontal distance traveled. For example, AFI11-202V3 requires a 200 feet/nm climb (all engines operating). If there are 6076.1 feet in 1 nautical mile, then 200/6076.1 = 0.0329. Hence, the regulated minimum climb gradient of 3.3%. Climb Gradient (2): (Boeing) While the Performance Manual does state that “climb gradient” is distance vs. height, this number, given by your FSAS computer or performance charts, represents the “takeoff and acceleration and climb capability of the airplane,” and is “used as a normalizing parameter to simplify Part 3 performance charts.” Maybe Boeing should have used a different term like “Climb Factor” to label this parameter. Nevertheless, think of the climb gradient given by the box or the charts as a measure of thrust excess… i.e. how well your jet will perform on a given day under specific conditions (temp, PA, gross weight, etc). How to compute climbout flightpath using charts… (geek method) 1. Determine the actual climb gradient from your FSAS computer or

performance charts (the climb gradient from your TOLD sheet or what the box is reading… i.e. 8.0%)

2. Determine whether or not you have a “close-in obstacle” (0 to 8,000 feet) or “distant obstacle” (8,000 to 40,000 feet). Remember… there are 6076 feet per nautical mile! For obstacles greater than 40,000 feet, (or about 6.5 miles) you must perform an enroute climb problem along with a takeoff climb problem.

3. Determine your Unstick to Obstacle Distance (UOD). This is a fancy way of saying, “how far is the thing you don’t want to hit from the point where my jet leaves the ground and starts to climb?) UOD = (Runway Available – Lineup Distance – Critical Field Length) + (obstacle distance from end of runway). If you were a Poly Sci major and can’t do math, and you want to “be conservative,” blow off this calculation and simply use the obstacle’s published distance from the end of the runway as the UOD.

4. Open your performance manual to the appropriate chart… they start on page 1A3-65. Be sure to pick the right chart for your flap setting and takeoff mode!

5. Enter the respective chart at the bottom with the UOD you previously calculated. Move up the chart to the climb gradient given by your FSAS computer. Move right to read your “Height Above Unstick.”

6. The performance manual’s minimum clearance for an obstacle is 1% of the obstacle distance from end of the runway. Add the minimum clearance requirement to the obstacle height and compare it to your height above unstick to determine if a safe takeoff may be accomplished

Note: If you are trying to clear an obstacle, you’re most likely flying a max mode profile… so don’t forget that clean-up height is 2000 feet AGL or 120 percent of the obstacle height above unstick, whichever is higher. What to do about a variable runway gradient? All of the performance data charts used to determine height above unstick have correction charts corresponding to your particular takeoff profile. How to make sure I don’t hit nothin’… (pilot method using the FSAS) 1. Check your NOTAMS, AP1, IFR Sup, Approach Plate, etc. for published

obstacle information or required climb gradient. Realize that performance charts deal with Unstick to Obstacle Distance (UOD) while your FSAS (both on the jet and on your desktop calculator) use Obstacle Distance from the end of the runway (OD).

2. Published obstacle information will typically depict an obstruction height in feet and a distance from the end of the runway in nautical miles. Once you convert the Obstacle Distance (OD) from NM to feet, you’re ready to plug and chug! (Remember: 6076 feet per nautical mile!)

3. Published Technique: On the bottom, left-hand side of your Takeoff Runway Data 1/7 Page, there is a spot for you to insert your obstacle information (distance and height, as expressed in feet… [OD/H]. After you have run your data, (assuming it is a “GO”) you will see the height at which you will clear the obstacle on your Takeoff Data 5/7 Page… about half-way down on the right side of the CDU. It is labeled “DHT” and stands for Delta Height Above Obstacle, up to 999 feet. Question: So what if the box gives me a “Takeoff NO-GO” because it determines that I can’t clear the obstacle? Good question… the box does not tell you how close you are to clearing the obstacle. Rather, it simply tells you whether or not a safe takeoff can be made. Therefore, try this technique…

4. Recommended Technique: Simply enter the Obstacle Distance and one foot on the Takeoff Runway Data 1/7 Page. For example, if you had a 300 foot tower at 1nm, enter [6076/1] for OD/H. The box will then tell you how many feet you will be over this “one foot” obstacle. It is then up to you how much clearance you’ll accept… remember, the Dash1 recommends 1% of OBSTACLE DISTANCE as an acceptable margin of HEIGHT to clear an obstacle. This technique also works great for determining your crossing height at the departure end of the runway (for complying with Army and Civil field procedures). You can use this technique to determine your height at various distances from the end of the runway and derive a “climb gradient” to compare with published SID climb requirements.

For reference use only!!! Questions or comments, contact Capt Don Kennedy, 55ARS, [email protected]

kc-135r/t climb gradient

Documents