in which the temperature increases with increasing altitude. A gas balloon flying in this inversion has the advantage of increased stability as compared to the altitudes above or below the inversion which exhibit normal lapse rates. 

To explain this, it is important to understand the term “stability” with respect to a gas balloon. Stability can be imagined as an invisible hand that gently pulls the balloon down whenever it starts to rise or alternately pushes the balloon back up as it starts to fall. A balloon flying in stable weather tends to fly level with very little intervention from the pilot. However, stability is a weak condition and can be overcome by many factors, such as gain or loss of solar heating, orographic winds, and ballasting or valving. 

To understand why an inversion creates stable flying conditions, think of a gas balloon flying at 3,000 feet in the middle of the inversion. If, for some reason, the balloon starts to ascend, two things happen. First, the balloon enters warmer ambient air. Second, the lifting gas inside the balloon expands and cools adiabatically as it reaches the slightly lower pressure atmosphere of the higher altitude. Both of these effects cause the balloon to lose lift and to descend. It is helpful to remember that, as a hot air balloon either cools or enters hotter air, the temperature differential between outside and inside air decreases and lift is lost. The same principle applies here, as applied to gas balloons. Subsequently, if the gas balloon descends from its starting point, it encounters cooler outside ambient air and its interior gas compresses and warms adiabatically, therefore gaining and ascending back to its original altitude. Remember, in an inversion, either motion (ascending or descending) tends to cause an opposing force to passively initiate a return to the original altitude. 

With proper planning, the gas balloon pilot can take advantage of this scenario. Weather theory teaches that inversions often set up at night either right at the surface (sometimes referred to as “surface inversion”) or at some altitude above the ground (generally referred to as an “inversion aloft”) as shown in Figure 11-8 . Visible signs of an inversion may be pollution trapped below the inversion causing reduced visibility and dirty looking air. Invisible signs of an inversion may be significantly more stable flying conditions in the inversion area. If a slow, steady initial ascent is initiated (best done by launching with a flaccid balloon), the balloon may find an inversion with no help from the pilot by leveling off as it enters the inversion zone. More likely the pilot has to hunt for the inversion; inversion levels can be determined from the use of the Skew-T charts as previously discussed in Chapter 4, Weather Theory and Reports. 

As an example, during flight at night very near the ground, a pilot may feel he or she is continually ballasting to fly 

level. If a pilot suspects an inversion aloft may exist above, he or she can initiate a slow ascent. If the envelope is flaccid and the ambient air at this altitude is not inverted, then the balloon tends to continue rising until it either encounters an inversion or becomes full as it reaches its pressure ceiling. If the balloon levels out while it is still flaccid, this may indicate that it has entered an inversion zone. If the balloon continues to fly level passively, it is flying in an inversion. 

## The Practice of Gas Ballooning 

## Gas Balloon Regulations 

For a pilot with a hot air rating, certification to fly gas balloons requires the removal of the “with airborne heater” limitation on his or her certificate. For a private pilot gas rating, aeronautical experience requirements in Title 14 of the Code of Federal Regulations (14 CFR) section 61.109(h) (1) require “…at least two flights of at least 2 hours each that consists of 

(i) At least one training flight with an authorized instructor within 60 days prior to application for the rating on the areas of operation for a gas balloon; 

(ii) At least one flight performing the duties of pilot in command in a gas balloon with an authorized instructor; and 

(iii) At least one flight involving a controlled ascent to 3,000 feet above the launch site. ” 

The regulation for removal of the airborne heater restriction from an existing hot air pilot’s certificate (14 CFR Section 61.115 Balloon ratings: Limitations) states: 

(2) The limitation may be removed when the person obtains the required aeronautical experience in a gas balloon and receives a logbook endorsement from an authorized instructor who attests to the person’s accomplishment of the required aeronautical experience and ability to satisfactorily operate a gas balloon. 

NOTE: Only a logbook entry is required to complete the process (no check ride), since the gas authorization is a removal of a restriction from an existing rating rather than an issuance of a new rating. 

The regulations for the aeronautical experience for a commercial rating are virtually identical according to 14 CFR section 61.129(h)(4)(i) except that the student must act as pilot in command on both flights and the controlled ascent must be to “…5,000 feet above the launch site.” 

