canaero - Canadian aviation history

An engineering perspective

Report by Raymon J. Kaduck

The conversion of the ATR42 was not merely cosmetic. It required an integrated approach that could solve a number of specific issues for northern carriers. The aircraft had to be a capable freighter, but the company refused to sacrifice cabin esthetics for function. It had to have a side aisle cargo compartment, but that meant incorporating a new fire suppression system and providing smoke protection. It had to be capable of bulk loading, so an integrated restraint system had to be designed, and even the movable bulkhead became a structural part of the compartment.


The liner of the cargo section needed to serve a number of purposes and First Air settled on a fibre metal laminate (FML) composed of two sheets of aluminum sandwiching a layer of fibreglass, only 58/1000th of an inch thick. Comtek Advanced Structures of Burlington, Ontario adapted it from a product that had been developed for ballistics use. While First Air has the overall STC for the conversion, Comtek holds the one for the liner material itself.

ATR42 interior without liners

The fibreglass in the FML greatly increases puncture resistance over an equivalent thickness of sheet metal. The fire rating is miles above what a sheet metal would be,” says Phil Lee, First Air Mechanical Designer. “Normally, cargo liners are fibreglass, but we needed ours to be structural. Also, if there is a puncture, it can be patched in place, using standard sheet metal procedures.”


The cargo liner has to provide much of the strength to meet 1.5g loading requirements of the cargo compartment, but it also has to be smoke proof. Liner sections have tongue and groove edges that seal so that only the movable bulkhead has to be taped to prevent smoke penetration.


The individual liner sections are specific to locations in the aircraft because they have to connect with the rest of the restraint system. In practice, First Air hasn’t attempted to swap sections between aircraft, although they are identical.


The requirement for a C Class compartment arises because of design changes necessary for the combi role. The existing ATR cargo compartment is a B Class, meaning that in the event of a fire, a crew member would don a protective suit and fight the fire. This is possible because the cargo compartment has a centre aisle wide enough to move in and allow quick access to all cargo.


First Air ATR42 CAD drawing

While this standard layout is convenient in the event of a fire, it is very wasteful. The highest section of the aircraft is used for the aisle, which sacrifices useful bulk space. The cargo areas are limited because the aisle itself is 25 inches, which is the distance between the inboard seat tracks. Finally, the factory design keeps the overhead bins in place, which limits the height of freight stored there to 49 inches.


“You just couldn’t carry the sort of cargo we do with a centre aisle,” says Lee. “A side aisle compartment was one of our primary design requirements.”


First Air’s design is considerably more efficient. It uses a side aisle 18 inches wide, making better use of the fuselage cross-section. The overhead bins are removed in the cargo compartment. The cargo area available for wide or bulky loads is considerably enhanced.


The tradeoff is that Transport Canada required a C Class compartment. This means that the fire suppression system is operated from the cockpit automatically and does not require a crew member to enter the compartment to fight the fire. The Kidde fire suppression system uses Halon bottles located under the floor panels. “They provide protection for 63 minutes, which could be increased with more Halon,” says Lee. |

ATR42 cargo section

Click on any photo for captions.