|Aircraft Hangar High Expansion Foam Incident|
Issue 88: Aircraft Hangar High Expansion Foam Incident
By Samuel S. Dannaway, PE, FSFPE
The entire hangar floor was covered with approximately 17 ft (5 m) of HEF (High Expansion Foam) engulfing all but the very top of the vertical fins of the A-10, F-16 and three F-15 aircraft ...
The deceased, age 31, and three other contractor employees, entered the hangar as they "were ‘curious’ to see the foam”. At some point, they became immersed in foam. The cause of death was not published. It may have resulted from extreme disorientation caused when immersed in the foam.
The investigation reported three findings. This article will focus on issues raised with finding #3 related to mitigation measures to "greatly reduce the potential for similar incidents …” Specifically, this article will address measures related to the design and criteria.
The system activated at 11:24 am. The accidental system operation was the result of a hangar wet pipe sprinkler waterflow alarm switch activation after a valve in the inspector's test froze and burst, causing a flow of water.
Because of this incident, and several other accidental system operations in hangars worldwide, the Air Force is conducting a global review of existing hangar HEF systems.
Not all accidental system discharges are caused by the operation of a sprinkler waterflow switch. In addition to the waterflow switch, there are two other common causes of accidental discharges. The first is operation of the manual foam release station and the second is a fault with the HEF control panel or its field devices.
The manual release station can be the culprit in a system release that is either accidentally or maliciously intended. Accidental releases, caused by bumping into or hitting the release station, can be mitigated by the use of a tamper proof cover. Many years ago, an AFFF system discharged when a sailor mopping the floor struck the release station. The use of a double-action station can limit accidental operation. Air Force guidance2 requires a tamperproof cover on the release station, which is, in effect, a dual-action function.
Accidental operation of the manual release station can also occur when it is mistaken for a fire alarm system manual station. The station color and signage requirements in Air Force guidance2 distinguish foam release stations from fire alarm pull stations. Personnel must be routinely instructed on actions to take in the event of a fire and the situations that indicate the need for operation of the foam system release.
The problem with HEF control panel faults is more controversial. The environment in hangars is often not controlled, and temperature and humidity may at times fall outside of the listed ranges of fire alarm system electronics. Additionally, water wreaks havoc with fire alarm systems when it gets into raceways and devices, and water in hangars is a fact of life. Rain happens; roofs leak.
Another issue is that addressable fire alarm system control units may not be stable enough to control fire suppression discharge. The Air National Guard has come to a similar conclusion3 about addressable systems. The requirements for hangar protection call for a conventional releasing panel.
15.6.2. The HEF panel shall be a non-addressable unit having an output capability, which emulates the inputs on a zone-by-zone basis.3
Air Force guidance2 requires equipping waterflow alarm switches with a retard feature to minimize the chance of pressure surges causing switch activation. The guidance also requires surge arrestors on sprinkler and foam systems to reduce the impact of pressure surges.
Foam discharge can also be accidentally initiated if an unsuspecting sprinkler technician operates the inspector’s test not realizing that foam discharge will occur. To mitigate this, designs can incorporate a plug in the wet pipe sprinkler test connection outlet with warning signage at each test valve.
To protect high value aircraft, one must detect and deliver fire suppression agent quickly and efficiently. Losses can be substantial in an aircraft hangar containing several F-22s, at a price of $150 million each, F-35s at $200 million, or a B-2 bomber at $800 million.
FM Data Sheet 7-93N4 cites Federal Aviation Administration testing in which failure of aircraft fuselage skin occurs within 45 seconds of a flammable liquid fire exposure. A Naval Research Lab Study,5 reported damage to aircraft adjacent (30 feet [9 m] away) to an aircraft involved in a fuel spill fire would likely not occur if total time to suppression (detection time + activation time + suppression time) is less than 110 seconds. This need to suppress the fire quickly must be balanced against the possibility for false activations.
One could use optical (flame) detectors and arrange the sequence of operations so automatic HEF discharge only occurs with one or more flame detectors activating in conjunction with the sprinkler waterflow alarm. Flame detectors will not cause any increase in detection time beyond that already expected with ceiling sprinklers. To convert HEF systems to manual activation only will only solve the problem with waterflow switch activation, while placing high value/mission essential aircraft at risk.
Another, more significant change would be to change the system from HEF to AFFF using Navy style grate nozzles. This system has the benefit of not producing inundating blankets of foam, but does bring with it increased costs in terms of higher water flow rates and water storage quantity, trench drains, and the possible need to deal with AFFF solution disposal.
The loss of life in the Eglin incident is unfortunate and tragic and correctly serves to highlight the need to address accidental HEF system activations. However, it should not result in removal high expansion foam from the design toolbox.
Samuel Dannaway is with S. S. Dannaway Associates, Inc.
4th Quarter 2013 -- Industrial Fire Protection – Jonathan M. Eisenberg, Rolf Jensen & Associates, Inc.
1st Quarter 2013 -- Challenges of Aircraft Hangar Fire Protection: The Development and Use of a Modern Standard -- Michael E. Aaron, P.E., Rolf Jensen & Associates
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