Recovery of an aircraft? Airbags to the rescue!
Published: 01 October, 2005
Recovering an aircraft is not an exercise for the weak-hearted. It is a specialised job that requires skill and training. “Every recovery is different and therefore requires a different approach,” explains Jan Pieter Maarschalk. “The type of aircraft and its position all influence the way the recovery should be handled. Weather and ground conditions also have a major impact. These factors influence the grip the tools have and how easy - or not - it is to position them correctly.”
Commonly-used tools are cranes, hydraulic jacks and large, low pressure airbags. Trucks or anchor points are used to stabilise the aircraft. Usually the recovery begins with airbags, as cranes are unavailable or have problem reaching the right spot.
“When lifting with cranes, the wings need to be partly dismantled creating an additional problem. Airbags are used because there is little entry room and the bags only put a minor amount of stress on the aircraft’s structure,” says Jan Pieter.
“The CATCH-22 behind recovering an aircraft is the fact that it is a heavy object but can only withstand a small amount of stress (point load) on its structure. Only on a few dedicated areas, the so-called ‘jacking points’, can it take a high load; these areas are meant for the hydraulic jacks,” he says. “Unfortunately these are rarely accessible when an aircraft is on its side.” Airbags have a low insertion height and distribute the lifting capacity over a large area. Depending on the type of aircraft, large stacks of these bags need to be created in order to reach the necessary height,” he reports.
The downside of the airbags is that they are not very stable - due to their low-pressure. An especially large stack is very unstable, causing problems for the recovery teams or fire brigade personnel. The stability issue behind the airbags is one of the main reasons why recovery is a complex and time-consuming job.
In some large, modern aircraft types this stability problem is even greater as the wings have a more pronounced v-shape than in the past. One of the reasons for this is that these passenger jets have larger diameter, ‘high-bypass’ turbofans than earlier models.
To accommodate these power plants, aircraft need to have a sharper angle between fuselage and wing. This causes serious problems for low-pressure airbags: the bags need to conform to the shape of the wing, when placed between two non-parallel surfaces (e.g. ground and wing) a large stack of these mattress-shaped bags can buckle out quite easily. During lifting the angle between wing and ground increases even more making it more difficult for the airbags to ‘follow the load’.
“For some years now, products designed for smaller aircraft - such as the NT bags from RESQTEC Zumro, a high pressure rescue bag that combines a high-lifting capacity and the ability to be connected for stable lifting - have been employed,” says Jan Pieter.
Unfortunately, the NT bags alone however are not able to exert a lift above 2.1 metres and have a reduced lift capacity (to limit pressure on the plane) of 6.5 metric tons (7.2 tons US). They are currently used by airports to lift the smaller aircraft.
Demand from airports and recovery organisations has encouraged RESQTEC to develop a full size system that enables teams to lift large commercial aircraft, both old and new types. The system is called R2S (short for Rapid Recovery Solution).
Fred Baas, Senior Engineer and Project Leader of the R2S project reveals it was not an easy task to find this solution:
“We had to overcome several problems when lifting at heights above 2.1 metres. The biggest problem was stability of a high column of NT bags’. The second problem was finding the solution for the changing ‘angle of attack’ (a.o.a) and the shifting of the lifting point during lifting.
“At the same time, feedback from the industry showed us that the system needed to be rapidly deployed, be small and lightweight and flexible enough to accommodate the lifting of different aircraft types.”
Jan Pieter comments: “Airports which already use the NT bags only need to buy additional modules instead of a complete new system. Three years, several prototypes and many trials later we are confident that this product fits all the needs from recovery teams all over the world,” he says.
So how does it look?
The basis of the R2S system is the RESQTEC NT-Bag, a round, high-pressure (10 bar /145 PSI), high-performance lifting bag. The NT bags were primarily developed for rescue operations and combine a modest size with huge lifting power: the NT-132 bag that the R2S system is based on has a maximum lifting capacity of 132 metric tons (145 tons US). The NT bags can be coupled together which allows users to create a column.
The simplest and smallest form - one already in use by airports for some time - is the R2S-1 which comprises three connected NT 132 bags where the top bag is inflated with just 0.5 bar (7.2 PSI) allowing the bag to mould itself to the skin of the aircraft.
This system can only be used for small aircraft (lift capacity is limited to 6.5 metric tons (7.2 tons US) with a maximum height of 1.4 metres/4.7 ft.
The R2S system for larger planes consists of a stack of 4 to 6 NT bags, stabilised by a frame to overcome the stability issues of a stack of more than three bags.
The system is designed to lift a load of 15 metric tons (16.5 tons US) up to a height of 3.9 metres/12.8 ft per column. For larger loads more columns are used. Columns are kept small for optimum placement and to avoid sharp objects. To avoid damage to the fuselage or wing skins, a column of high-pressure bags is topped with a large low-pressure bag, which is supported by a lightweight platform. The thickness of the low pressure bag is kept to a minimum to insure maximum stability.
Fred Baas explains: “The beauty of the whole system is that all the parts can be assembled prior to lifting. Assembly is straightforward, fast and easy. All components of the frame are connected with self-locking pins. When the NT bags inflate (one at the time), the frame will expand and stabilise itself as soon as the vertical tension straps are stressed.”
“The first delivery of the full R2S system is scheduled for November-December 2005. More than 20 airports currently use the simple version of the R2S system and have already given commitments to upgrading their systems to the full size version,” concludes Jan Pieter.