Amateur built hot air balloons
Guidance for safely designing, constructing, and operating amateur-built hot air balloons in the UK.
The UK is a major manufacturing centre for hot air balloons. Since the late 1960s, the UK has been at the forefront of hot air balloon design and construction. In recent times a community of amateur built hot air balloons has emerged, based on historic designs, and using provisions in aviation regulation that allow aircraft, where at least 51% of the fabrication and assembly tasks are performed by an amateur, to be designed and constructed outside of a regulated environment. New designs, that in some cases incorporate novel shapes and features, are now emerging.
This web page provides guidance to amateur builders on the safe construction and operation of balloons. Amateur constructed aircraft in the UK fall within the definition of non-Part 21 aircraft.
The introduction of these webpages is in part as a result of the fatal accident involving amateur built balloon G-CMFS at Ombersley Court, Worcestershire on 25 June 2023. AAIB subsequently made a safety recommendation 2024-008 to the CAA and these webpages are in response to that recommendation, alongside the amended Safety Directive 2021-004R2.
Envelopes
General
Design codes and certification specifications have been developed in the UK, mainland Europe and USA. In 2001, a working group was convened by EASA to take the best aspects of existing codes and specifications, and consolidate them into a single European design code, CS-31HB. This is the current design code for all UK balloon manufacturers.
Since its inception, CS-31HB has been shown to be an effective method of ensuring hot air balloon designs are airworthy and should be the starting point for any home builder.
It is not expected that home build designs demonstrate compliance with recognised balloon design codes, but relevant codes should inform the builder of construction standards, conventions (such as control line colours and loads) and best practices that should be incorporated in the design. Particular regard should be given to the design of the envelope to minimise the probability of parachute stall, paying close attention to the requirements contained in CS-31HB.
Construction materials
On initial review, the list of requirements in CS-31HB may seem onerous. However, many of them can be satisfied by using similar construction materials that are used by existing balloon manufacturers. For example, it may prove easier to purchase load tape from a manufacturer (or their supplier) rather than researching and sourcing an alternative. The builder should establish a maximum operating temperature for the envelope based on the materials used. A temperature flag with a compatible melting link should be fitted to ensure the envelope does not overheat.
Manufacturing methods
Manufacturing methods (stitching, splicing, knots etc) are well documented in the Instructions for Continuing Airworthiness issued by hot air balloon manufacturers. Novel methods of construction should be assessed and verified before use.
Maintenance of the sewing machine, and selection of compatible thread and needles is essential. Nomex (or equivalent) thread should be used in areas subject to elevated temperatures.
Inspection
It is strongly recommended that the builder commissions an independent inspection once the build is complete. Inspections should check all areas of the balloon and be documented for future reference.
Flight testing
Hot Air Balloons are unique in the fact that most testing can be carried out a few feet from the ground with little risk to the pilot. The functioning of control systems should be checked with the balloon inflated. It is good practise to assess performance through a range of masses to ensure safe operation. Once the initial performance has been verified, a programme of testing should be carried out to establish the operating limitations of the design. In most forms of aviation, flight testing is carried out by experienced individuals. Builders should consider using an experienced test pilot if they intend to prioritise performance. The operational characteristics of parachute stall should be reviewed during the flight testing.
Pilot’s notes
Once operating limitations have been established, the builder should make a note of the safe limits for weather limitations, rates of climb and decent and maximum time for parachute opening.
Record keeping
The builder should create a build file where records of the design, materials used, inspection and testing should be stored.
Fuel systems
In all cases it is recommended that the fuel system used is from certified manufacturers and that pressurised fuel systems, including gas containers, are regularly inspected to ensure safety both in the air and on the ground to minimise the risk to third parties. All elements of the fuel system should be capable of being released to service in accordance with certified continuing airworthiness requirements.
Baskets and chairs
If baskets from a balloon manufacturer are used, they should be eligible for release to service in accordance with certified continuing airworthiness requirements.
If a builder intends to construct their own basket, it is recommended that traditional methods are used and structural components such as suspension wires are sourced from reputable suppliers. Termination of suspension wires should be done in accordance with standard industry practises using proprietary tooling maintained in accordance with the tooling manufacturers requirements.
If novel construction methods are used, CS31HB provides guidance for construction and testing.
Maintenance (continuing airworthiness)
It is recommended that amateur built envelopes should be inspected after the first few flights, after six months and then annually. If it is expected that the balloon will fly more than 100 hours a year, an hours-based maintenance regime should be adopted, with an appropriate calendar-based back-up. Fuel system and other bottom end components should be inspected in accordance with the original manufacturer’s instructions. Home built baskets should be inspected to a schedule appropriate to their design. Records of inspections should be kept.
The inspections should be carried out by a competent person. Even if the owner/builder feels they have the competence to carry out an inspection, it is recommended that the inspection is carried out by somebody independent.
Mandatory Occurrence Reports (MORs)
Although occurrence reporting for non-Part 21 aircraft (which includes amateur constructed aircraft) is not mandatory in the UK, it is recommended that reportable occurrences are processed through the Occurrence Reporting system to share safety information.
Safety reporting and Just Culture
Change of ownership
It should be understood by all parties that the responsibility for the design and construction of any amateur built assemblies remains with the original builder and designer. Comprehensive records of design, construction, testing and maintenance should be transferred to the new owner.
Pilot's notes and original limitations should be explained to the new owner. The new owner should also be made aware of any defects that have not been rectified.