1. Aircraft Dynamic Response
ROUGH RUNWAY ANALYSIS
Aircraft Dynamic Response
Jack Hagelin
Boeing Technical Fellow
ASTM E-17 Seminar
December 5, 2006

2. Types of Runway Roughness Impact on Aircraft
Three Types of Runway Roughness Impact on Aircraft:
Limit Loads – Single Discrete Bump Events
Fatigue Loads – Continuous Long Wavelength Bumps
Truck Pivot Joint – Continuous Short Wavelength Bumps
Each type imposes a different runway roughness criteria.
Current standards address mainly first two types.
Third type is not directly addressed in current standards.

3. Limit Load Dynamic Taxi Analysis
Limit Load Critical Conditions
Constant speed taxi
Accelerated Takeoff
Decelerated Landing Rollout
Ride Comfort
Runway profile used as direct input
Goal is to use roughest runway reasonably expected in normal operation (FAR ).
In absence of other data, AC provides SF28R profile data.
Analyze both forward and reverse directions, and multiple tracks if available.
Discrete bump conditions also analyzed (Boeing single bump and FAR double bumps)
Runway crown effect for airplanes with more than two main gears
Important input parameters:
Airplane weight, cg, pitch inertia, speed, aerodynamic forces, and thrust
Landing gear location (wheelbase), number of gears, shock strut air curve, tire stiffness, brake force
Tail aerodynamic force affects nose gear load
Airplane on-the-ground natural frequencies (usually from 0.5 to 1.5 Hz)
Important output parameters:
Landing gear vertical loads, airplane cg acceleration, forward fuselage, and engine loads

4. Bump Height vs. Length Diagram FAA Certification Runway (AC 25
Bump Height vs. Length Diagram FAA Certification Runway (AC ) – SFO 28R 3 2 1

5. Limit Load Dynamic Taxi Analysis Results – San Francisco (SFO) Runway 28R

6. Bump Height vs. Length Diagram Sample Recently Measured CIS Rough Runway

7. Sample Rough Runway 200m section with critical bump
Smooth line is curve fit used to flatten profile

8. 3D Power Spectral Density (PSD) Plot Sample CIS Rough Runway
Airplane Speed of 60 knots 1 2 3

9. Limit Load Dynamic Taxi Analysis Sample CIS Rough Runway

10. Bump Height vs. Length Diagrams Russian Certification Runways (A, B, C, D)

11. 3D PSD Diagrams Russian Certification Runways (A, B, C, D)

12. Limit Load Dynamic Taxi Analysis CG Vertical Acceleration Comparison
Limit Load Factor = 1.7g per AC

13. Fatigue Dynamic Analysis
Fatigue Critical Conditions
Preflight taxi
Takeoff roll
Landing rollout
Post-flight taxi
Runway profile used as direct input
‘Typical’ runway/taxiway profiles used to match in-service statistical data
Important input parameters:
Airplane weight, cg, pitch inertia, speed, aerodynamic forces, and thrust
Landing gear wheelbase, number of gears, shock strut air curve, tire stiffness
Airplane on-the-ground natural frequencies (usually from 0.5 to 1.5 Hz)
Important output parameters:
Landing gear vertical loads
Airplane cg acceleration
Engine loads
Fuselage, wing, and horizontal tail incremental loads

14. Sample Taxiway which may be rough for Fatigue
Critical Taxi Speed = 24 knots for 1 Hz airplane response

15. Main Gear Pivot Joint Analysis
Current runway standards do not adequately address this issue
On rough runways, the bogie pitch mode exhibits low damped resonant response at 10 to 20 Hz range due to short wavelength roughness (~2 to 7 m)
Important parameters for pivot joint analysis:
Pivot pin diameter
Friction coefficient in joint
Bearing pressure due to post load
Bearing surface velocity
PV (Pressure * Velocity)
Heat energy generated in the joint
Time duration of heating

16. Pivot Joint Heat Calculations
Pressure-Velocity =
Friction Energy = FA
F = Post Load FF
Bogie Pivot Joint
F / 2
Inner Cylinder
Bogie Beam R
Pivot Pin
m = friction coefficient at bushing surface

17. Short Wavelength Runway Profile Analysis Power Spectral Density (Overall Runway)
Shows bump height intensity versus bump wavelength
Acceptable
Threshold

18. Runway Profile Analysis 3D Relative Power Spectral Density
Acceptable
1500m
Relative Runway Height PSD (dB)
Runway Distance (m)
Rough 2 7
Bump Wavelength (m)

19. Runway Profile Analysis RMS of PSD Sorted by Airport

20. Pivot Joint Dynamic Takeoff Analysis Results – Typical International Runway

21. Pivot Joint Dynamic Takeoff Analysis Results – Borderline Runway

22. Pivot Joint Dynamic Takeoff Analysis Results – Rough Runway

23. Dynamic Analysis Results –Takeoff Roll Pivot Joint Heat per Unit Bearing Area (Normalized) vs. Runway

24. Dynamic Analysis Runway RMS vs. Pivot Joint Heat

25. Dynamic Analysis CG Vertical Load Factor vs. Pivot Joint Heat
Correlation of CG Acceleration with Pivot Joint Heat is not strong.
(Due to widely different response frequencies)

26. Summary
Three types of airplane response to runway roughness require three different acceptance criteria.
Airplane dynamic analysis is necessary to evaluate effects of rough runways on aircraft design, however, runway profile analysis can be used to determine acceptable roughness levels.
For limit loads, single bump height vs. length chart is good criteria. (This criteria is not adequate for fatigue or pivot joint).
For fatigue loads, a PSD criteria may be used in the 10 to 75 m range.
For pivot joint, PSD approach works well (2 to 7 m range).
RMS value in this range is a good measure of pivot joint impact.
Work is in progress with FAA to establish short wavelength runway roughness standard to address pivot joint friction.