CS 31TGB.21 Loads

ED Decision 2013/011/R

Strength requirements are specified in terms of:

(a) limit loads that are the maximum loads to be expected in service, taking into account the load factors of CS 31TGB.23; and

(b) ultimate loads that are limit loads multiplied by factors of safety defined in CS 31TGB.25.

CS 31TGB.22 In-Service load cases

ED Decision 2013/011/R

The strength requirements include consideration of the applicable in-service load cases such as:

               inflation;

               flight; and

               mooring.

The loads are determined and the parts and components under particular stress designed in accordance with their designated use and dimensioned such as not to fail under recurrent loads.

AMC1 31TGB.22 In-Service load cases

ED Decision 2013/011/R

Inflation/mooring

The ‘inflation and mooring cases’ referred to in this requirement cover assembly, disassembly, inflation, deflation and mooring load cases. Mooring load cases cover both low and high mooring, if applicable.

When the balloon is moored in the parking position (low mooring) the maximum gas pressure in the envelope is normally identical to the ‘maximum gas pressure’ established for any of the flight conditions. If the low mooring operation, however, allows for a precautionary increased pressure of the gas in the envelope this load case is also considered.

Flight

Flight load cases cover the operation within the established limitations (temperature, wind speed, mass, and ascent/descent speed limitations). A dynamic lift component is considered in the load cases for the sudden deceleration of the ascent/descent unit and when the envelope shape is not spherical and generates lift in wind conditions. When a dynamic lift component is applicable, gust loads are considered as well as potential oscillation behaviour of the balloon and the tether caused by airflow and from variations in the lift component and its centre of pressure.

CS 31TGB.23 Load factors

ED Decision 2013/011/R

(a) Flight load factor. In determining limit load, the flight load factor is at least 1·4, except for (b).

(b) Ascent load factor. In determining limit load on the tether system, the ascent load factor appropriate for the type of operation is established and applied to the static load cases on the tether system.

(c) Gust load factor. In determining limit load, the gust load factor is established for the effect of gusts as defined in the operating limitations.

AMC1 31TGB.23(b) Ascent load factors

ED Decision 2013/011/R

The ascent load factor is applied to the static tether system load to cover dynamic loads to the tether system resulting from decelerations during the ascent. The maximum deceleration typically occurs when an emergency stop is made during maximum ascent speed. The highest loads are typically experienced when this occurs at maximum static lift and minimum balloon weight and minimum deceleration travel. Minimum balloon weight and minimum deceleration travel coincide at low tether cable length when the mass of the tether cable is the lowest and the elongation or slack of the tether cable are the lowest.

For an ascent speed below 1 m/sec, an ascent load factor of 2 is acceptable.

AMC1 31TGB.23(c) Gust load factor

ED Decision 2013/011/R

A gust load factor is applicable to balloons that due to the shape of the envelope generate aerodynamic lift forces in gust conditions. The gust load for spherical balloons is, therefore, 1 and is considered to have no influence on the loads.

CS 31TGB.25 Factors of safety

ED Decision 2013/011/R

(a) A factor of safety is used in the balloon system design as provided in the table.

(b) Regardless of the materials used, the load-bearing components of the suspension and tethering system is designed so that failure of any single component will not jeopardise safety of flight, and that total failure is extremely remote. Account is taken of any reasonably foreseeable dynamic or asymmetric loading affects associated with the initial element's failure.

(c) Where no provision is made for duplication in the suspension or tether system, the factor of safety is to be multiplied by a factor of 1·5.

(d) For design purposes, an occupant mass of at least 77 kg is assumed.

AMC1 31TGB.25(b) Factors of safety

ED Decision 2013/011/R

The dynamic loads on a balloon system are difficult to evaluate because metal or textile parts behave quite different.

In absence of a more suitable method or as replacement of a load test, the failure of the load bearing component shall be shown by the following method:

Multiply the limit load in the failing load path by two and distribute it as a static load among the remaining load paths.

For conventional designs, this is an appropriate method which is based on good service experience.

CS 31TGB.27 Strength and proof of strength

ED Decision 2013/011/R

(a) The structure is able to support limit loads without permanent deformation or other detrimental effects.

(b) The structure is able to withstand ultimate loads for at least 3 seconds without failure.

(c) Proof of strength of the envelope material and other critical design features are tested.

(d) The gondola is of a generally robust design and provides the occupants adequate protection during a hard landing.

(e) The design and strength of components also considers the effects of recurrent and other loads experienced during transportation, ground handling, and mooring.

(f) The effect of temperature and other environmental conditions that may affect strength of the balloon is accounted for.

AMC1 31TGB.27(c) Strength and proof of strength

ED Decision 2013/011/R

The envelope tests may be performed on representative portions of the envelope provided the dimensions of these portions are sufficiently large to include critical design features and details such as critical seams, joints, load-attachment points, net mesh, etc. Also refer to CS 31TGB.45 for specific tear propagation requirements.

AMC1 31TGB.27(e) Strength and proof of strength

ED Decision 2013/011/R

The strength requirements need to include consideration of loads during transport, ground handling and rigging. The loads need to be determined and the parts and components need to be designed in accordance with their designated use and dimensioned such as not to fail under recurrent loads.