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14CFR31.85 ("Required Basic Equipment" for manned free-air balloons) requires an Envelope Temperature Indicator (ETI). This is a requirement of the FAA, not necessarily the manufacturer. Some balloon Flight Manuals do not even mention an ETI.

My question is, is there somewhere that explains why the FAA mandated a ETI for all hot air balloons, and how it's used with regard to the determination of load, overheating and gross lift with a hot-air balloon? (Might be more than one question! ;-))

There does not seem to be an industry standard, and the temperature inside a HAB is critical to flight performance and safety. Best regards, FFG

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    $\begingroup$ Welcome to Av.SE $\endgroup$
    – Ralph J ♦
    Jan 16, 2019 at 15:19
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    $\begingroup$ "and the temperature inside a HAB is critical to flight performance and safety" - Might that possibly be why the FAA mandates an ETI? $\endgroup$
    – Vikki
    Sep 6, 2019 at 0:43
  • $\begingroup$ Is it to do with ice forming on the envelope? $\endgroup$
    – pericynthion
    Sep 6, 2019 at 4:22
  • $\begingroup$ the balloon handbook which you should be keeping in your balloon, has a chart that shows lift/payload based on ambient and internal temp at the sensor. plus your log book shows the weight of the equipment, and in flight school you learned the weight of propane, so you can easily compute the spare payload you have to carry for the temp shown in the weather report plus the max you want or can run the envelope safely. thats how you use it with regard to determination of load $\endgroup$
    – old_timer
    Jan 18, 2020 at 15:30
  • $\begingroup$ the sensor is critical to safety as well as flight operations. idealy you shouldnt hit max temp or be close to it. at the same time you dont carry a scale and weigh each passenger, but before launching you do check for equalibrium and can check the temp then which is the same as weighing everything, flight time will be hotter than that but with experience you get a feel for how close to the margin you are $\endgroup$
    – old_timer
    Jan 18, 2020 at 15:32

2 Answers 2

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A hot air balloon (including the air in the envelope) has tremendous mass, so it takes a long time for a change in buoyancy to cause a change in vertical speed. Watching for temperature changes on the Envelope Temperature Indicator will help a pilot anticipate changes in the balloon's vertical speed before they have actually had time to occur.

From page 7-4 of the FAA's Balloon Flying Handbook:

"Ascents and Descents

The temperature of the air inside the envelope controls balloon altitude. A balloon that is neither ascending nor descending is in equilibrium. To cause the balloon to ascend, increase the temperature of the air inside the envelope. If the temperature is increased just a little, the balloon seeks an altitude only a little higher and/or climb at a very slow rate. If the temperature is increased significantly, the balloon seeks a much higher altitude and/or climbs faster. If the balloon is allowed to cool or hot air is vented, the balloon descends.

Even without the input action by the pilot, it must be remembered that the air inside the envelope is dynamic. The air mass is constantly moving within the confines of the envelope, attempting to seek a level of equalization. While it varies with each envelope, input action by the balloon pilot can take from 6 to 15 seconds to be realized as a reaction by the balloon. Planning the maneuver, anticipating the reaction time, inputting the proper burn, and observing the reaction must result in smooth and natural movement by the pilot."

Also, as another related answer has noted, under some conditions it is possible for a pilot to bring the envelope to an unsafe temperature by excessive operation of the burner. Monitoring the Envelope Temperature Indicator allows this to be avoided. For a given desired maximum envelope temperature, the outside air temperature determines the total gross lift that is available at any given density altitude, or the density altitude that can be reached with a given gross weight. See the FAA's Balloon Flying Handbook for more.

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I know nothing about balloons but the FAA's Balloon Flying Handbook mentions at least two reasons.

First, it can be a limitation from the manufacturer:

Limitations. Restrictions placed on a balloon by its manufacturer. Examples are maximum envelope temperature and maximum gross weight

I don't know exactly why the limitation would be there, but I assume it's a material safety issue at some point.

Second, it's used for performance calculations to determine lift. The Handbook has a diagram on p. 3-9 and the explanation says:

Using the chart in Figure 3-13, determine the maximum gross lift that may be expected on a 60 °F day, with decisions not to exceed 190 °F envelope temperature and 1,500 feet pressure altitude. In this example, the established parameters equate what many pilots consider when doing performance planning. They decide they do not want to exceed a given altitude or envelope temperature.) To determine the maximum gross lift available, the nomograph should be entered at point A, at the ambient temperature of 60 °F. Move right, to the line indicating an envelope temperature of 190 °F (point B). Then, move down vertically to a point equidistant between the lines denoting altitude of 1,000 and 2,000 feet (point C). Then, move horizontally to the left to the gross lift axis of the chart and read the result (point D). In the illustrated example, this computation results in a maximum gross lift of 1,120 pounds.

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