Printed headline: Rethinking Cargo
In aircraft design, weight issues have long ranked first among engineering concerns, while payload has long been a central criterion for a customer’s choice. One aircraft niche seems to be evolving with volumetric capacity as a key design driver—cargo conversion.
As e-commerce grows, in addition to helping the conversion market thrive, it is making the fuselage interior’s volume more important than payload, since a parcel most often contains a light object tucked in fluffy protective packaging.
One unchanging factor has been low aircraft utilization for cargo services, meaning transforming a passenger airliner into a freighter generally remains the only economical option, rather than purchasing a purpose-built cargo aircraft.
Nowadays, “the aircraft will cube out before it grosses out,” says Brian McCarthy, vice president, marketing and sales with Precision Aircraft Solutions, a specialist in such conversions. The density of cargo is low, he explains, at a typical 6.0-6.5-lb./ft.3. “Integrators and e-commerce giants are shipping ‘bubble wrap and boxes’, and you can see this as you look at the typical package you get at home.”
Asked about the latest trends in the cargo market, Gilbert Birke, head of sales, Airbus Freighter Conversions with EFW (an ST Engineering Aerospace joint venture with Airbus) sees three. First, e-commerce is making delivery lead times more challenging. Second, the lower cargo density translates into demand being less payload-driven and more volume-driven, he says, concurring with Precision Aircraft’s McCarthy. Third, cargo is being increasingly containerized both on the main deck and in the lower cargo hold in a bid to minimize the turnaround time—with 2 hr. being a maximum, he suggests.
The e-commerce industry also has an impact on the size of aircraft that are in demand. The medium-large segment, represented by the Boeing 767, provides the required volume, according to an IAI Aviation Group representative.
The delivery process has always been time-sensitive. Moreover, operators are paid by the pallet or container position. So there are some constants when designing a cargo conversion.
McCarthy sees “loads, stress, damage tolerance and fatigue studies” coming first. Modern commercial aircraft are designed and optimized for the passenger transport mission. “Lightweight structures are used, which are basically not designed to carry the heavy loads required for cargo operations,” EFW’s Birke adds.
Therefore, the main focus of the conversion is on strengthening the airframe—especially the main deck’s floor grid—using partial reinforcements and the installation of new and stronger structural parts and assemblies, says the IAI source. The company’s approach in the medium and large segments is to replace the main floor structure. In the small aircraft segment, it reinforces the existing structure.
One more design driver is the requirement for a wide center-of-gravity envelope, the IAI representative says.
Additional major changes in a conversion include the cargo handling system, which uses motorized wheels integrated in the floor panels. The environmental control system—which manages cabin pressure and air conditioning—also has to meet different requirements than for a passenger aircraft. Smoke and fire detection equipment is omnipresent.
Just behind the flight deck, the crash barrier is a special bulkhead that can withstand a 9g deceleration while keeping the cargo from sliding through. There is also the main cargo door. Sometimes, the passenger version’s door is replaced by a crew entry door.
Precision Aircraft Solutions has experience with Boeing aircraft and is developing its first Airbus conversion, for the A321. “Airbus airplanes are optimized structures already. They are lightweight, [and] they are very efficient structures. So when you put a cargo door on [one], it takes a sophisticated approach and the latest engineering tools to substantiate the alterations and stay within the envelopes,” says McCarthy. “We must be very vigilant to introduce the new load paths into the existing structures. Airbus models are more difficult to convert, but at the end of the day will be more flight- and fuel-efficient.”
Does the weight of the reinforcing structures tend to outweigh component removals, thus making the empty weight of the converted cargo aircraft higher than that of the original version? That is true for EFW’s A330P2F—1-2 metric tons heavier, depending on the aircraft variant. But the situation is more favorable for EFW’s A320 and A321 conversions, according to Birke.
Similarly, on Precision Aircraft’s A321, passenger cabin equipment (which is removed) weighs more than the cargo handling system, flight deck storage modules (which include baggage space and a galley), main cargo door and crash barrier, McCarthy says. They account for approximately 1,600 lb., 225 lb., 800 lb. and 700 lb., respectively.
As a result, the freighter is several thousand pounds lighter than the passenger version. The weight difference is said to be similar to Precision Aircraft’s Boeing 757 conversion.
The time it takes to convert an aircraft is counted in months—“between four and six months, depending on aircraft type and adjacently performed base maintenance and nonconversion-related modifications,” says EFW’s Birke. For Precision Aircraft’s A321 and 757, that duration is about three months and 3.7 months, respectively. This includes heavy maintenance that operators usually choose to perform simultaneously, says McCarthy.
A typical cargo conversion project at IAI takes about four months, including maintenance work and additional modifications. “Most of the customers use the grounding time of the conversion to perform a heavy check and replacements of engines or other equipment,” says the representative.
Precision Aircraft is targeting April or May 2020 for the concurrent FAA-European Aviation Safety Agency certification of its A321 conversion. A daunting task was to design a finite element model for the airframe conversion process. It was crucial for the engineers to have an adequate data set, which is also based on other analysis tools and instrumented flight tests.
“We purchased an A321 fuselage, disassembled it and analyzed it,” says McCarthy. Because Airbus competes with Precision Aircraft via EFW, the U.S. company did not rely on the OEM to generate its loads or data set.
“We develop our loads and data sets from our principles,” says McCarthy. Airbus has pledged to support the unmodified structures and systems. For the rest, “it is our responsibility, as [supplemental type certificate] holder, to maintain continuing airworthiness,” he adds.
The company says its A321 conversion offers a unique capability— a flight deck able to accommodate six occupants. Precision Aircraft believes it will give it an edge over EFW’s conversion, as both offer 14 container positions on the main deck. This gives the operator the ability to move more personnel—pilots and mechanics—while allowing its pilots more flexibility to position within the network.
Precision Aircraft’s trick is the longitudinal orientation of the last container in the back of the fuselage, a layout that makes the most of the available volume. The configuration also retains the original passenger door, as opposed to EFW’s smaller crew hatch.
Payload and internal volume are targeted at 27 metric tons and 8,000 ft.3, respectively. For an 89-metric-ton maximum takeoff weight with maximum payload, range will stand at 2,030 nm. The company plans on converting 10 A321s per year.
“Engineering tools have leveled the playing field between OEM and non-OEM conversion programs,” says McCarthy.
Portland, Oregon-based Precision Aircraft began in 2002 with developing a Boeing 757-200 conversion. It was certified in 2005. Since then, 115 have been delivered to 17 operators, including DHL and Chinese carriers.
The production rate stands at 11 757s per year, including sites in the U.S. and China. McCarthy predicts this rate will decrease in 4-5 years, despite ample feedstock. There are an estimated 200 757s with fewer than 28,000 cycles, which makes them conversion candidates, thanks to the 50,000-cycle potential (known as limit of validity), says McCarthy.
EFW, based in Dresden, Germany, says it provides “a clean integration of the freighter-specific features into the OEM’s cockpit computer system, which makes the converted freighter feel and look more like a factory freighter than a converted one.”
It offers A320, A321 and A330 conversions. For the widebody, a powered cargo loading system with a smart control logic simplifies loading and unloading and reduces the labor required, says Birke.
IAI, based in Israel, holds supplemental type certificates for Boeing 737, 767 and 747 conversions. It also has production sites in Mexico and China. In 2019, it expects to convert about 15 767s and 5-6 737s, in addition to receiving the certification for the 737-800 conversion.
About 80% of Amazon’s fleet is 767s converted by IAI, notes the representative.
For 767s, a good candidate is a 15-30-year-old aircraft, with midlife engines, auxiliary power unit and landing gears. Such an aircraft may cost $10-12 million, IAI says. Even adding about $14 million for the conversion and $1.5 million for a heavy check, the resulting $25 million or so compares favorably to a new-build freighter, which may cost 5-6 times more, the IAI representative asserts.
However, in McCarthy’s view, a new widebody may make more sense than a new narrowbody, thanks to the longer range and the demand for higher reliability.