Re: DC-9 Hail damage

Date:         29 May 98 02:44:13 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>> Outer panes of the windshield were cracked. By the way, this
>> incident reminds me of a very similar incident with Southern
>> Airways in April 1977 -- a DC-9 approaching Atlanta encountered
>> a thunderstorm not far away from Calhoun, Ga; but both engines
>> failed and would not restart. The pilot got the aircraft onto
>> a highway, but there were 70 fatalities.

>I seem to recall that it was a 727 ...

Nope.  Southern never had any 727s as far as I know.  In any case, the
accident in question, which took place on April 4, 1977, involved a
DC-9-31, registration N1335U.  Incidentally, most or all of the
fatalities occurred because the port wingtip caught an embankment just
after touching down on the highway, and this caused the plane to veer
to the left off the highway, right into a gasoline station.  The impact
killed both pilots and 31 passengers; another 20 passengers died in the
fire.  Eight people on the ground also died.

>The fatal damage was not due to
>the hail hitting the plane, although all the leading edges were quite
>dented, but rather the sheer volume of water ingested by the
>engines. The flight crew focused on restarting the engines, which had
>all puked all their turbine blades out the back.

It was actually somewhat more complex than that.  The fan and first five
low-pressure compressor stages showed no significant signs of foreign
object damage, and tests showed that high water ingestion rates merely
caused reduction in rotor RPMs without damage.

However, it was discovered that at low RPMs, water ingestion caused the
high-pressure compressor to become much more sensitive to stalls and
surges, a situation exacerbated by throttle advancement.

The NTSB's conclusion was that the engines were reduced to low power
settings, probably to slow the aircraft to turbulence penetration speed.
The subsequent advancement of the throttles, in combination with the
high water ingestion rate, caused the high-pressure compressors to
surge.  The overpressure caused the blades of the sixth (and final) low-
pressure compressor stage to deflect *forward* to impact the fifth-stage
stator vanes.  Fragments of these blades and vanes were ingested into
the high-pressure compressor, causing severe damage.  Continued high
power settings resulted in high fuel flow, which in conjunction with
reduced compressor efficiency led to excessive turbine temperatures,
which in turn finally caused the failure of the engines.

(Most of this from Macarthur Job's Air Disaster, volume 2.)

--
Karl Swartz	|Home	kls@chicago.com
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