Northwest Airlines Icing related problem IAD Jan 27 6:30pm EST

Date:         30 Jan 97 00:36:26 
From:         shahid siddiqi <s.a.siddiqi@larc.nasa.gov>
Organization: as&m
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 Hi newsgroupers I was flying out of National yesterday at about 6:15 pm
EST in a Mooney (general aviation single engine airplane) and we heard
the following exchange on the Washington Control frequency, if anyone
has details please pass on to newsgroup:

A Northwest Heavy (don't know type of airplane, must have been a DC-10
or 747) called requesting lower altitude because de-icing could not be
accomplished.  Controller cleared them for 7000 ft.  Then the pilot
requested 5000 and a place to dump fuel so as to return to Dulles to be
able to land.
The whole exchance was calm on both sides, I think an emergency was
declared and 13,000 lbs (I think that's what I heard) of fuel (almost
2,000 gals Wow that's $1,200 enough of a fuel bill to do my Mooney for
10 flights @ $2 /gal Av GAs)
The dumping was done near Gaithersburd MD and the controller warned all
traffic to stay away from the area because of fuel in the air.

Two questions for the conesenti:
1 - Icing was a problem yesterday above 8,000 feet in the Washington
area that's why I was cruising at 5,000.  Had the 747 dumped fuel above
a cloud layer with icing  would that have affected, at least for some
time, the icing hazard in that cloud for other airplanes?

2 - How long does the Jet A take to evaporate and not be a hazard if
some other airplane comes by. (Hazard as in take an airliner takes in
the air mixed with kerosene into its cabin air inlet as well as a small
airplene like mine flying through it and perhaps injesting it into the
reciprocating engine and having a detonation problem)