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From: wjb@hpfcso.HP.COM ( Bill Buse )
Newsgroups: rec.aviation
Subject: MD80 Ice Revisited
Message-ID: <7760036@hpfcso.HP.COM>
Date: Sat, 13 Oct 90 21:11:56 PDT
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 78

I received the information I'd been waiting for concerning the MD80 and
ice forming on the wings as a result of cold fuel in the wing tanks.

It has been determined that if a wing containing a full or nearly full
fuel tank is exposed to the cold of high-altitude cruise for a long
period, it becomes cold-soaked.  If the aircraft subsequently lands at
an airport with an ambient temperature above freezing and a readily
available source of moisture such as rain, drizzle, fog, etc., that
moisture will freeze when it comes in contact with the top surface of
the cold-soaked wing.  

This ice that forms is usually clear, and because the entire surface
of the wing, icy and non-icy areas alike, is often wet and shiny, the 
ice is difficult to see.  This is particularly true in the poor lighting
conditions usually accompanying rainy days and especially during rainy,
early morning walkarounds.  

In most cases it is not possible to confirm ice by visual means alone;
the only way to know for sure is to feel the surface with a hand or with
an ice-sensitive device. 

If ice is there but not detected, the natural flexing of the wing during
the taxi for takeoff may loosen the ice, and at rotation the air flow over
the wing may cause it to break loose.  If it has formed in the area ahead
of the engine, as is often the case, the ice flies into the engine intake
lip/bullet/guide vanes, etc., shattering into smaller pieces as it passes
into the fan section, creating the soft foreign object damage (FOD) damage
long associated with ice. 

Why the increase of cold-soak ice and engine FOD with the MD80?  There are
some important factors:

1. The greatly increased supply of fuel in the MD80 has made 3, 4, and even
   5 hour flights commonplace, so airframes and wing fuel get thoroughly
   cold-soaked on a regular basis.  

2. The increased capacity of the center fuel tank of the MD80 in comparison
   to the DC-9, from 3,000 kg (6,000 lb) to nearly 10,000 kg (20,000 lb)
   delays the consumption of wing fuel.  Dispatched with a full center tank,
   an MD80 may fly for 3 hours before that tank supply is exhausted.  If the
   aircraft then lands before most of its wing fuel is consumed, the stage
   is set; all that is needed are cool temperatures for wing ice.

3. In the MD80, the bulkhead that forms the divider between the center tank
   and wing tank is about 5 feet out from the fuselage.  The ice typically
   begins to form at this divider, above the wing tank and outboard of this
   bulkhead, beginning near the back of the wing where the aft spar forms 
   the rear bulkhead of the fuel tank.  Ice in this area on an MD80 is almost
   directly in front of the engine intake, and ice forming outboard of this
   area may also be swept into the engine by the prevailing local air flow. 

Ambient ground temperatures higher than 60 F (15 C) may still permit cold-soak
ice to form.  This cold-soak ice has been reported in warm places such as
Las Vegas, Palm Springs, and Grand Cayman Island.   

To help with the visual inspection of this ice, a service bulletin from
Douglas calls for the installation of four triangular decals with tufts on
each wing.  The triangular decals are located just outboard of the main
tank bulkhead in four locations.  They are located aft of the front spar,
midway between the front and aft spar, forward of the aft spar, and the
last one between the main tank access panel and the aft spar.  The latter
two locations are the most critical because ice develops in these areas
first and then spreads forward and outboard.  

Checking the tuft ends for freedom of movement provides a good indication
of wing ice.  If the tuft ends are not free to move, wing upper surface
ice is presumed to be present.  

Bill Buse 

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