Kein Folientitel

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Meteorological Investigation of an Accident on
December 2001 with a Britten Norman /
BN-2B-26
Udo Busch, Klaus Sturm, Herbert Leykauf, Erland
Lorenzen Deutsche Wetterdienst,
Offenbach Business Unit Aviation
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Presentation

• Introduction
• Synoptic situation
• Snow and ice cover on the aircraft
• Conclusion

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Introduction

• In Germany the BFU (German Federal Bureau of
  Aircraft Accidents
• Investigation) is authorized to investigate
  accidents and serious incidents
• to civil aircraft in Germany, to determine the
  causes of the occurrences.
• The German National Meteorological Service (DWD)
  is responsible for
• the meteorological part of the investigation. The
  results of these
• determinations form the basis for safety
  recommendations and advisory
• notices, statistical analyses, research, safety
  studies and ultimately
• accident prevention programs.

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Synoptic situation

• Observed at the weather station Bremerhaven
• 0824 0844 UTC mod to severe snow shower
• 0844 0905 UTC light rain shower
• 0900 UTC snow cover (wet snow) hight 0.5 to
  1.5 cm
• 0913 UTC time of accident no significant
  weather

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Synoptic situation at the time of the accident

• measurements at the airport Bremerhaven
• and in the vicinity after snow and rain showers
  are passing
• no significant weather
• visibility between 30 and 40 km
• air temperature between 0.7 and 6 C
• dewpoint temperature between -3 and -0.1 C
• westerly wind with 09 kt
• spread radiosounding Emden (00 and 12 UTC) 3 - 4
  K

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0845
0830
0915
0900
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Snow and ice cover on the aircraft

• Presented is one example of our bulk
  calculations
• The estimation of the minimum airspeed to
  separate
• the wet snow cover from the aircraft

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adhesion of a wet snow cover on the
aircraft estimation of surface stress, dynamic
pressure and adhesion
Fa Mo x G G h x b x t x s x
g Ff ½ x rho x v2 x Af x cw, (frontal
area) Fs ux 2 x rho x b x t (surface) ux
u (z) x kappa / ln (z/z0) K kappa / ln
(z/z0), z 0.10 m, z0 10-4 m ? K 0,05
Fs U(z)2 x K2 x rho x b x t
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adhesion of a wet snow cover on the aircraft
F Ff Fs v2 x
rho x h x b x t x (K2 / h ½ cw/t) Mo x G
v2 x rho x h x b x t x (K2 / h ½ cw/t)
v ( (Mo x s x g) / (rho x (K2 /
h ½ cw/t)) )0.5
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adhesion of a wet snow cover on the aircraft
with cw 0,2 , Mo 0,74 (Steel on Steel), rho
1,25 kg/m3 and h 1 cm, t 1 m minimum
airspeed v 90 m/s or v 180 kt with
h 0.5 cm, t 0.2 m minimum airspeed v
54 m/s or v 108 kt
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Results
The minimum airspeed which was necessary to
separate the wet snow cover from the aircraft was
between v 108 kt and v 180 kt. The
estimation of possible frost or freeze on snow on
the aircraft wings within the slipstream of the
propeller (calculation with basic evaporation
and ventilation equations) shows that between 2
and 5 minutes frost or freeze on snow was
possible. The weight increase as a consequence
of the snow cover was between 75 kg and 225
kg. Carburetor icing was possible.
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Conclusion
The pilot attemped to brush the snow only from
the wing-nose, not from the whole aircraft. He
decided to go with a snow or ice cover of
approximate 1 cm. Once airborne, the aircraft
stalled and crashed into a river approximate 1 km
behind the runway. 8 persons died and one was
seriously injured. Investigations of comparable
accidents (e.g. the Potamac accident,
Washington 82) show, that a snow or ice cover
of only 0.25 cm results in a loss of lift of
approximate 30. After take-off it was
impossible to keep the aircraft in a stable
flight attitude.
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Possible frost or freeze on snow on the
aircraft The estimation is only for the thin
water film on the wings within the slipstream of
the propeller
S B L V 0 (energy equation of a wet
surface) q x mw V x A x t
(q335x103 J/kg) V 0.622 x Lv x a L x
(E - eL) / (p x cp) a L 38,5 x (v / d)0.5
with Td 2C and T 0,7 C 335 x103 W
s/kg x (0,1 kg x 0,4 m2) 120 W/m2 x 0,4 m2
x t 335 00 W s
120 W x t s ? t 279 s oder t 4
minutes and 39 sec.