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Fire extinction in hangars. Keys for its suitable design

José Alfonso López

José Alfonso López

AERTEC / Airport Planning & Design

 

The international NFPA 409 hangar fire protection standard tends to be applied fairly extensively – not only in Spain, but also around the world – given that it is the most comprehensive specific standard on hangars that exists today.

The standard sets some highly precise design criteria for the building and its facilities, differentiating them on the basis of the maximum fire area and the size of the aircraft it is meant to house.

NFPA 409-based fire protection for hangars is a de facto standard giving the designer very little margin. However, the proper choice of extinction systems is the key to safe hangars.

Said design criteria leave the designer with no margin with regard to their application, except for the fire extinguishing systems, where three alternatives are possible:

  1. Closed-head foam-water sprinkler system.
  2. Automatic sprinkler system with a low-level low expansion foam system.
  3. Automatic sprinkler system with a low-level high expansion foam system.

At first sight, the choice of one or other system may seem to be of minor importance, but nothing could be further from the truth.

The closed-head foam-water sprinkler system is usually the most suitable option since it requires a single system that reduces costs, especially if the wing area of parked aircraft does not require a support system below the wings (low or high expansion foam systems).

The problem arises when maintenance tasks are carried out that leave the aircraft’s electrical and electronic systems exposed. AFFF foam concentrates, which are the most commonly used, can cause damage to said elements since none of the technical authorities of the said products’ manufacturers have confirmed and validated this fact in writing.

This is further reinforced when we review the specific US Department of Defence standards on hangars, where such option is not foreseen.

Thus, if the closed-head foam-water sprinkler system is discarded due to the maintenance tasks carried out on the aircraft, which of the two low-level systems is more suitable? Each system has advantages and disadvantages which the designer has to weigh up.

In low-level low expansion foam systems, the foam is delivered either through oscillating monitors or ground nozzles.

The former suffer from the disadvantage of discharge pattern obstacles, particularly near the monitors. It is thus highly important to make workers aware that they should not hinder the way in which the oscillating monitors work. In any case, the aircraft themselves can become obstacles in hangars of a certain size housing a large number of small aircraft. This kind of equipment is therefore not always the most suitable. Ground nozzles should be used in this case.

The disadvantage of ground nozzles resides in their elevated costs. They are patented equipment which is only supplied by a single manufacturer in the market. The entire surface area of the hangar has to be covered by a grid of these elements and the branches have to be left accessible for maintenance purposes. This means that the flooring or ground slab (depending on the aircraft) becomes much more expensive, over and above the accessible channels’ costs. This cost rises as the size of aircraft increases.

The US Department of Defence’s UFC 4-211-01N is a reference document which specifies the use of a solution made up of an automatic sprinkler system with a low-level low expansion foam system using ground nozzles for hangars designed to house small aircraft.

In low-level high expansion foam systems, the foam is delivered by foam generators located at ground level on the building’s walls, which have to be fed with external air to generate the foam.

The disadvantage of this system has to do with the hangar’s operations, given that the hangar doors have to remain closed, otherwise the foam generated would escape outside without achieving its aim.

A possible solution to this problem would consists of closing the hangar doors automatically in case of fire to ensure they remain closed while the low-level high expansion foam system is activated. A suitable emergency power feed would therefore be necessary.

The Department of Defence’s ETL 02-15 is a reference document which specifies the above-mentioned solution for Air Force hangars.

As can be seen, installing the most suitable fire protection system depends on each specific case. The designer must therefore assess which solution adapts itself best to his/her needs.

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