If the flight is continued through sunset, loss of solar heating results in a cooling of the lifting gas and a resultant loss of gross lift. Ballast must then be used to maintain buoyancy. 

## Purity of Lifting Gas 

A final topic of interest is the effect of mixing air with the lifting gas. It may seem that since air is heavier than the lifting gas, the air would have a tendency to pool at the bottom of the envelope and be expelled through the appendix as the balloon rises into less dense air. However, this is not what happens. The forces of molecular attractions cause the air and lifting gas to become permanently mixed and a generally less pure mixture occurs. When this happens, some of the benefits of operating under the pressure ceiling are lost and every up/down maneuver causes the loss of lifting gas along with the expelled air. 

For this reason, care should be taken to close the appendix when descending rapidly to avoid allowing air to force its way up into the envelope through the open appendix. Maintaining a high level of purity of the gas inside the envelope can extend flight duration. Care must be taken to open the appendix during rapid or prolonged ascents which bring the balloon close to its pressure ceiling. Ascent above pressure ceiling with a closed appendix increases internal pressure on the envelope’s fabric and, in an extreme case, could cause the envelope to rupture. 

## Landing, Retrieval and Packup 

The landing phase is the single most critical portion of the entire flight. The pilots may be fatigued from the long flight. They may be in unfamiliar geography and weather conditions. There may be time pressure to land in daylight if sunset is approaching. A landing after dark, under a full moon and in open territory may be performed rather routinely, but a daylight landing is still much preferred. A night time landing in dense woods under a new moon or with reduced visibility can be very stressful. Five minutes on oxygen prior to landing may help to relieve some fatigue and clear the senses. 

The landing decision varies with each flight and should be discussed jointly among all the pilots. Landings should be initiated while there is still adequate ballast available to abort at least one approach, if necessary. The actual amount of ballast required varies with pilot experience, weather conditions, terrain and descent rate. For a 1,000 cubic meter balloon in relatively easy landing conditions and under a shallow descent, 50 pounds may suffice. Under more adverse conditions, 250 pounds or more may be advisable. Additional considerations for landing may include duration of flight, fatigue level of the pilots, accomplishment of the flight objectives, current and forecast weather, terrain, and time of day. 

When the decision is made to land, all possible equipment should be securely stowed. Occupants should don helmets and any other protective gear. All antennas, solar panels, and other items hanging below the basket should be retrieved and stowed. Adequate ballast should be brought inside the basket where it is readily available to abort or round-out a landing. 

The trail rope should be rigged and readied for deployment. The trail rope serves three purposes. First, it slows the descent rate. A fully deployed trail rope weighing about 40 pounds on a 1,000 cubic meter balloon normally arrests a descent of approximately 340 feet per minute (fpm) at 150 feet above ground level (AGL) to a descent rate of 0 fpm at ground contact. The descent from 150 feet will take about 45 seconds. These numbers are only approximations and local conditions at landing certainly cause these to vary somewhat. Secondly, the trail rope orients the balloon so the attachment point of the rope is on the upwind (or trailing) side of the basket. This may be important depending on the arrangement of the basket and/or deflation ports. Finally, as more of the rope contacts the ground, friction slows the horizontal speed of the balloon. The trail rope should be connected to the load ring with a quick release mechanism to allow release should the trail rope become permanently entangled on the ground. 

If the gondola has rotated during flight, placing the trail rope on the gondola’s downwind side, it does not deploy correctly unless the pilot guides it around to the upwind side of the gondola. Otherwise the rope deploys under the gondola and tends to pull the leading edge of the gondola down and may completely invert it. This is colloquially called “dog-housing” and puts the occupants in the uncomfortable position of being dragged along the ground trapped inside the basket. Unlike modern hot air balloons, most gas balloon systems do not have rigid uprights, so dog-housing is a real concern in a high wind landing; the best antidote is to keep as much weight as possible (including the occupants) on the upwind (trailing) side of the basket. 

The trail rope also acts to stop any ascent since a rising balloon becomes heavier as more of the rope is lifted off the ground. This is why it is sometimes called “retrievable” or “reusable” ballast. For this reason, the trail rope should never be deployed until landing is completely certain. Aborting a landing with a deployed trail rope requires ballasting the weight of the trail rope (approximately forty pounds) in addition to the ballast normally required to achieve the desired ascent rate. 

The anti-sail line should be pulled tight and secured to the load ring or other strong point. The purpose of this line is to hold the bottom of the envelope down taut to minimize 

