Abort point Figure 12-10.  A physical abort point on the runway should be determined and briefed to the glider pilot before starting the towing  procedures.  Since density altitude is perhaps the single most important  factor affecting airplane performance, a review of the effects  of density altitude is in order. An increase in air temperature  and/or humidity significantly decreases power output and  propeller efficiency. The engine produces power in proportion to the weight or  density of the air. Therefore, as air density decreases (high  density altitude), the power output of the engine decreases.  Also, the propeller produces thrust in proportion to the mass  of air being accelerated through the rotating blades. If the air  is less dense, propeller efficiency is decreased. The problem of high density altitude operation is compounded  by the fact that when the air is less dense, more engine power  and increased propeller efficiency are needed to overcome  the decreased lift efficiency of the tow plane’s wing. This  additional power and propeller efficiency are not available  under high density altitude conditions; consequently, tow  plane performance decreases considerably. The winds aloft should be continuously evaluated to help  determine the glider release area and always attempt to  release the glider upwind of the airport. Code of Federal  Regulations (CFR), part 91, section 91.309 (a)(5) states, “The  pilot of the towing aircraft and the glider have agreed upon a  general course of action, including takeoff and release signals,  airspeeds, and emergency procedures for each pilot.” If there  is any doubt that this rule has not been complied with through  briefings or published and agreed upon standard operating  procedures, ensure both you as the tow pilot and the glider  pilot are absolutely clear on all aspects of the upcoming tow. The tow pilot must develop an awareness of the direction of  the tow plane’s prop blast. Blasting launch personnel and  glider canopies with all sorts of debris is undesirable and  potentially dangerous. Whenever possible, turn away from  any ground operations. Prior to taking the active runway  for tow line hook-up and takeoff, if available, monitor and  announce your intentions on the Common Traffic Advisory  Frequency (CTAF). In most cases, the Standard American Soaring Signals are  used to communicate between the launch crew and tow plane.  In some cases, however, specific local procedures may be  in effect. The tow pilot should be thoroughly briefed on any  specific local procedures. The tow pilot may be required  to observe these signals through the mirror or through an  additional signal relay person positioned safely on the side  of the runway adjacent to the tow plane. The ground signals  are listed below and are also presented as illustrations in  Chapters 7 and 8 of this handbook. •  Take up slack—the take up slack signal is given by  the ground crewmember moving his or her lowered  arm from side to side. When you receive this signal,  slowly taxi the tow plane forward to take up the slack  in the tow line. When all the slack has been taken from  the tow line, expect to receive the “hold” signal from  the ground crew. •  Hold—this signal is given by holding the arms out  straight.  •  Pilot ready, wings level—when the glider pilot is ready  for takeoff, a thumbs up signal is given and the wing  runner will level the wing to the takeoff position. •  Begin takeoff—the glider pilot waggles the rudder  with the wings level and the wing runner signals  with a circular motion of the arm. When ready  for takeoff, the tow pilot should broadcast on the  CTAF that a glider launch is about to be initiated.  For example, “Tallahasee traffic, N12345 taking off  Runway 33, glider in tow, Tallahasee.” Remember,  14 CFR part 91, section 91.309, requires that before  conducting towing operations within Class B, C, D, or  E airspace designated for an airport, or before making  each towing flight within such controlled airspace if  On the Airport Ground Signals 12-6