Henry Hackford, Aerospace Technical Services Business Manager at Stirling Dynamics, provides a unique perspective on the passenger-to-freighter (P2F) conversion process.
With the rise of ageing fleets and newfound uncertainty over passenger numbers due to COVID-19, the inevitable time will come when operators face a difficult decision about what to do with assets that are not as commercially attractive as they once were. The lifecycle of a commercial aircraft can typically be extended by minor or major modifications to accommodate for a new role change. Converting a passenger aircraft to a freighter is an example of one of these changes. Passenger-to-freighter conversions are an attractive secondary prospect for older aircraft as the airfreight market consistently relies on a mix of newly built freighters and passenger aircraft conversions. According to Boeing’s World Air Cargo Forecast 2018-2037, over the next 20 years “2,650 freighters are forecast to be delivered, with approximately half replacing retiring aeroplanes and the remainder expanding the fleet to meet projected traffic growth. More than 63 percent of deliveries will be freighter conversions”.
The attractive proposition of a passenger-to-freighter conversion
If you are in possession of a passenger aircraft that is approaching the end of its operational service or is no longer commercially viable in its current arrangement, the proposition of converting it to a freighter aircraft, as opposed to scrapping it, is an attractive and economical prospect but it’s not a straightforward process. The conversion of a passenger aircraft into a freighter is a complex engineering challenge requiring specialist technical skill sets. This is a major programme and any changes to the original aircraft require detailed analysis of the proposed modifications along with the presentation of design evidence to the certification authorities for airworthiness clearance. Changes to the airframe structure and increased payloads all need to be considered, as well as the installation of new systems suitable for the aircraft’s new role all of which must comply with the rigorous certification process involved.
A surprisingly complex engineering challenge
From our perspective at Stirling, the technical complexity behind the scenes of a P2F conversion can be as involved as designing a new aircraft from scratch. A common misconception is that the conversion process is a physical process – simply stripping out the aircraft interior, installing a cargo door and plugging the windows – however, these physical changes are only possible after a great deal of critical analysis to ensure the flight safety of the modified airframe. The kind of work we get involved with includes, but is not limited to, the calculation of critical airframe loads from which the design will evolve. Other aspects Stirling is often asked to analyse are flutter analysis, systems integration and the impact of the modification on the landing gear.
When we talk about “loads” we are talking about the forces applied to the aircraft structure in both its normal and extreme operating conditions. This work is necessary as the aircraft will be taking on different payload distributions as a freighter and the resulting loads need to be analysed in-flight and on the ground, to provide evidence that the aircraft can cope safely in its new role.
“The technical complexity behind the scenes of a P2F conversion can be as involved as designing a new aircraft from scratch”.
Often, we have limited aircraft data available so must supplement this with in-situ measurements and reverse engineering methods to obtain the information we need to create representative simulation models. These models are then matched and validated against physical flight test measurements. Certification loads are then derived from flight and ground simulations to meet the relevant requirements. This data is passed onto the design team, who use it when designing the modifications to the aircraft. The loads work we do is an important stage in progressing the modification design.
In some specific cases, we can show compliance with certification requirements by demonstrating that the freighter modification does not produce loads that are more critical than the passenger version. We’ve developed in-house ground simulation tools over the years that are unique to Stirling to help us achieve this. We are one of the few independent companies in the world that can offer this experience and capability set. The conversion to freighters offers an ageing fleet a new lease of life, which is something we very much endorse in Stirling. It was predicted that P2F conversions would account for at least 60% of the freighter market, and given the situation we now find ourselves in, potentially more over the next 20 years. These conversions look to be heavily focused toward standard-body passenger jets and provide an effective solution in giving new life to old assets.
Stirling Dynamics offers a complete design and analysis service from concept to certification and has recently carried out loads analysis for the modification of the A321-200 passenger aircraft to a freighter variant with customer, Precision Aircraft Solutions.