What I am looking for here is a sort of a list gathering the manoeuvres and/or load cases imposed by regulation that lead to the sizing of a part of an aircraft. For instance, vertical gust load is often the load case that has to be used to size the structural parameters of the horizontal tail (bending moment). What are the other load cases and manoeuvres that usually size the subparts of a commercial aircraft ?


1 Answer 1


The applicable US regulation is FAR 25.301-563. This obviously goes into a lot of detail about the different aspects that should be taken into account. I will try to condense the cases as much as possible, but many are conditional and relate to various other conditions. Many cases include some subjectivity as to what the FAA will require. Every certification is unique.

While the requirements are somewhat broken down into the general areas of the plane, it's impossible to say exactly which areas of the plane will be critically affected by a certain case. The impact depends on the design and configuration of the specific plane.

There are the upper level requirements:

(a) The structure must be able to support limit loads without detrimental permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation.

(b) The structure must be able to support ultimate loads without failure for at least 3 seconds...

(e) The airplane must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to VD/MD, including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope.

Definitions of various important values (which will have structural implications):

VA - Design maneuvering speed
VB - Design speed for max gust intensity
VC - Design cruising speed
VD - Design dive speed
VDD - Design drag device speeds
VF - Desgin flap speeds
VMC - Minimum control speed with critical engine inoperative
VS0 - Stall speed with landing flaps
VS1 - Stall speed, flaps retracted
CN - Normal force coefficient

All strength requirements must be met at all places within the maneuvering envelope, particularly A-I on the following V-n diagram:

V-n diagram

  • Max pitch control deflection at VA (25.331)
  • Specified pitch control displacement (25.331)
  • 1.5g pullup (25.335)
  • Discrete gust, vertical and lateral (25.341)
    • VC: varies linearly
      • +/- 56.0 ft/sec - sea level
      • +/- 44.0 ft/sec - 15000 feet
      • +/- 26.0 ft/sec - 50000 feet
    • VD: 0.5 times VC values
  • Continuous gust, vertical and lateral (25.341)
  • Fuel/oil loads from zero to maximum (25.343)
    • Addtional factors if there is a "structural reserve" of fuel considered
  • High lift devices (25.345)
    • If flaps are to be used for takeoff, approach, or landing, limit load factor of 2.0
      • +/- 25 ft/sec normal to flight path
    • Separate conditions, limit load factor of 1.0
      • Propeller slipstream from max continuous power at VF
      • Propeller slipstream from takeoff power at >= 1.4 times VF and max weight
      • Head-on gust of 25 ft/sec
    • If flaps are to be used en route:
      • Maneuver to positive limit load factor
      • Discrete vertical gusts
    • Maneuver load factor of 1.5 g at max takeoff weight, flaps in landing configuration
  • Roll maneuvering (25.349)
    • Airplane load factor 0 and 2/3 of positive maneuvering factor
      • Steady rolling velocities
      • Maximum angular acceleration
      • VA Sudden deflection of aileron to the stop
      • VC Aileron deflection for same roll rate as VA case
      • VD Aileron deflection for at least 1/3 of VA case
    • Unsymmetrical gusts
  • Yaw maneuvering (25.351), speeds from VMC to VD
    • Sudden max rudder deflection (conditional definition)
    • At that max deflection:
      • Yaw to overswing sideslip angle
      • Yaw to static equilibrium sideslip angle
        • Rudder suddenly returned to neutral
  • Engine torque (25.361)
    • Limit engine torque at takeoff power and prop speed, 75% limit loads of V-n condition A
    • Limit engine torque at max continuous power and prop speed, 100% limit loads at V-n condition A
    • For turboprops, previous conditions with prop control system malfunction
    • For turbine engines:
      • Torque from sudden engine stoppage
      • Torque from max engine acceleration
  • Engine/APU side loads (25.363)
    • Limit side load factor
  • Pressurized compartments (25.365)
    • All flight loads in addition to pressure differential 0 to max relief valve setting
    • External pressure distribution in flight
    • Landing loads combine with max pressure allowed at landing
    • Pressure including safety factor for high altitude aircraft (>= 45,000 feet)
    • Sudden release of pressure for critical components
      • Also affects bulkheads, floors, and partitions
    • Other emergency depressurizations
  • Engine failure unsymmetrical loads (25.367)
    • Failure of critical engine (for turboprops, also failure of drag limiting system)
      • Speeds from VMC to VD, due to fuel flow interruption
      • Speeds from VMC to VC, due to mechanical failure
    • Includes pilot corrective action
  • Gyroscopic loads (25.371)
    • Must be considered at max appropriate RPM for certain other cases
  • Speed control devices (25.373)
    • Symmetrical and yaw maneuvers and gust conditions, each setting with max speed
      • If automatic/load limiting features, include corresponding speeds/positions
  • Control sufrace/system loads (25.391)
    • Considering other limit load cases
      • Loads parallel to hinge line
      • Pilot effort effects
      • Trim tab effects
      • Ground gust conditions
      • Unsymmetrical loads
      • Auxiliary aero surfaces
  • Landing load condtions (25.473)
    • Descent at 10 fps, design landing weight
    • Descent at 6 fps, design takeoff weight
    • Both cases should be considered in the following cases:
      • Level landing
      • Tail-down landing
      • One-gear landing
      • Side loads
    • Rebound landing
    • Ground handling
    • Taxi, takeoff, landing roll
    • Braked roll
    • Turning
    • Tail wheel yaw
    • Nose wheel yaw/steering
    • Pivoting
    • Reversed braking
    • Towing loads
    • Unsymmetrical ground loads
  • Seaplanes (25.521)
    • Step, bow, stern landings
    • Unsymmetrical landing
    • Hull/main float bottom pressures
    • Auxiliary float loads
    • Seawing loads
  • Emergency landing (25.561)
    • Occupants must have reasonable chance of escaping serious injury in minor crash landing if occupant experiences the following forces:
      • Upward 3.0g
      • Forward 9.0g
      • Sideward 3.0g on airframe, 4.0g on seats
      • Downward 6.0g
      • Rearward 1.5g
    • Large masses should not:
      • Cause injury to occupants
      • Penetrate fuel tanks
      • Block escape
    • Protection of occupants in dynamic conditions
    • Ditching strength
  • Fatigue and damage tolerance (25.571)
    • Residual strength with certain failed structure:
      • Limit symmetrical maneuvers
      • Limit gust conditions
      • Limit rolling conditions
      • Limit yaw conditions
      • Pressure differentials
      • Limit ground loads
    • Safe-life evaluation, if failure conditions are not practical
    • Sonic fatigue
    • Safe flight with damage from:
      • 4 pound bird at VC at sea level and 0.85VC at 8,000 feet
      • Uncontained fan blade impact
      • Uncontained engine failure
      • Uncontained high-energy rotating machinery failure

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