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Technical Note (National Research Council of Canada. Division of Building Research), 1970-10-01
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Ignition of Magnesium Alloy Components Following Aircraft Accidents
Williams-Leir, G.
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https://nrc-publications.canada.ca/eng/view/object/?id=5ce7f10b-3a09-4ad6-ba0b-bd199f7dc5ee https://publications-cnrc.canada.ca/fra/voir/objet/?id=5ce7f10b-3a09-4ad6-ba0b-bd199f7dc5ee
NATrONAL RESEARCH
COUNCIL
OF CANADA
DIVISION OF BUILDING RESEARCHN
({)'JfJE
No.
555..
,tltEPAR,£D _BY G. Williarrls - Leir CHECKED BY GWS APPROVED BY NBB
October 1970
l':lU:CPARED FOR Record Purposes
IGNITION OF MAGNESIUM ALLOY COMPONENTS FOLLDWING AIRCRAFT ACCIDENTS
In crashes of aircraft that incorporate components of magnesiurrl alloy, tJ:-J.ese cOlllponents may be ignited by fuel burning on the gr ound, or by other means. The Accident Investigation Division of the Department of Transport has requested inform.ation on the tilYle-scale of this process. It was hoped to throw light on the sequence of events leading to or
follow-ing a helicopter accident, where magnesium oxide was stated to have been drawn into a gas turbine that would be expected to run down in at most several seconds after impact.
To ignite bulk rrlagnesiurn it is not neeeSsary to heat it until ｩｧｮｩｴｩｏｬｾＮ
occurs. It is sufficient to raise it to a critical temperature est:.uated at 585°C1• From that point, oxygen diffuses to the m.etal and surface com-. bustion takes plac e sufficiently to cause com-.its temperature to continue to
rise, eventually reaching ignition at around 630° C. This ignition point
is slightly below the melting point 650° C, so that ignition is rapidly fonowed by melting. Time lags from critical te:mperature to ignition up to over thr ee hour s have been recorded2.
The present problem, however, is not how slowly, but rather how quickly magnesium can be ignited froni the sources envisaged. '1:,;,,15 the problem. is one of heat transfer, where the tem.perature of "the object under consideration is determ.ined by the balance between rate of access of heat
2
-References
American Society fo1' Testing and Materials: Specification B90-66.
Kubaschewski, O. and Hopkins, B. E. Oxidation of Metals and Alloys. Butterworth, London 1962, page 210.
2.5 to 3.5% 0.20% 0.6 to 1.4% Manganese Zinc Aluminum
I:mpurities subj ect to limits in the specification
Magnesiurrl Balance to 100%
Alloys with these metals are stated4 to have oxidation rates higher than pure magnesium at telnperatures (420 to 4600
C) below the critical. This may make little differenc e if the minirrmrrl 19nition time is governed by heat transfer froni flame to metal as suggested above.
Mellor, A. M. and GlassIT1an, 1. A physical criterion for Uletal ignition. Pyrodynamics,
l.
43-64 (1965).Ibid, p.
59.
In several trials the rrlinimurrl time to ignite a I-in. square frorrl I'oom temperature was 1 min. 10 sec., and for a 1/4-in. square, 25 sec. Addition of an inert heat sink would obviously lengthen these times, if it did not'prevent ignition.
The material used was Alloy AZ31B3 which consists of:
from the flames, and rate of loss, primarily by conduction, to other COlll-ponents in direct contact with the object.
It was thought that sirrlple experiments would be both quicker and rnore convincing than heat transfer calculations. For the heat source, it
fiCCITlS plausible to as sume that lio diffusion flame of spilt fuel would be hotter than a premixed propane-air flame from a Meker burner. The ob-ject was a piece cut frorrl 3/8 inch rnagnesium alloy plate to various sizes " from 1/4 in. square to 1 in. square. It was supported by a pair of leads
from a thermocouple in the piece of plate, so that the heat sink was mini-lnal ..
