After all inspections are complete, compute the takeoff performance for the tow plane using the approved POH. As a rule of thumb, the tow plane is normally airborne in about twice the computed distance. If your tow plane does not have takeoff performance charts, make a normal takeoff without the glider in tow and note the lift-off point. Use this physical point in lieu of computed takeoff data. Pay particular attention to your altitude over the departure end of the runway and the clearance of any barriers.
Using the computed takeoff data or actual takeoff point, choose a physical abort point on the runway. [Figure 12-10] Thoroughly brief the glider pilot on the abort point and abort procedures. If the tow plane is not off the ground by the chosen abort point, the glider should release, or be released, allowing the tow plane to accomplish a normal takeoff.
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.
On the Airport
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).