text: inter- 
  ference is common when the two airplanes are 
  in a trail position and stepped down. As shown 
  in figure 6.10, the single airplane in flight de- 
  velops upwash ahead of the wing and down- 
  wash behind and any restriction accorded the 
  flow can alter the distribution and magnitude of 
  the upwash and downwash. When the trailing 
  airplane is in close proximity aft and below the 
  leading airplane a mutual 
  interference takes 
  place betweetrthe two airplanes. The leading 
  airplane above will experience an effect which 
  would be somewhat similar to encountermg 
  ground effect, i.e., a reduction in induced drag, 
  a reduction in downwash at the tail, and a 
  change in pitching moment nose down. The 
  trailing airplane below will experience an effect 
  which is generally the opposite of the airplane 
  above. 
  In other words, the airplane below 
  will experience an increase in induced drag, an 
  increase in downwash at the tail, and a change 
  in pitching moment nose up. Thus, when 
  the airplanes are in close proximity, a definite 
  collision possibility exists because of the trim 
  change experienced by each airplane. The 
  magnitude of 
  the trim change is greatest 
  when the airplanes are operating at high lift 
  coefficients, e.g., low speed flight, and when 
  the airplanes are in close proximity. 