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135.293(a)(7)(iii) requires that pilots be tested on:

(iii) Operating in or near thunderstorms (including best penetrating altitudes), turbulent air (including clear air turbulence), icing, hail, and other potentially hazardous meteorological conditions;

So what are the best penetrating altitudes when forced to operate in a thunderstorm?

The most obvious answer is to stay out of it in the first place, but the reg requires us to come up with an answer....

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  • $\begingroup$ This is one of those regulations that on the surface appears to contradict another one: Intentionally penetrating a thunderstorm strikes me as a flagrant violation of 91.13 outside a few very specific cases like Hurricane Hunters. Looking at it another way though, if a commercial flight inadvertently blunders into a thunderstorm I'd hope my pilot remembers this part of their training :-) $\endgroup$ – voretaq7 Dec 25 '13 at 6:27
  • $\begingroup$ @voretaq7: Well, I don't think that it is a contradiction. I would assume that this is for penetrating a thunderstorm when you don't have a choice (ie the weather has closed in around you, most likely due to poor decision making in the first place). $\endgroup$ – Lnafziger Dec 25 '13 at 22:44
  • $\begingroup$ Personally I view that poor planning and decision making as the violation of 91.13 - but of course having said that, yes: it behooves you to be familiar with "what to do when it all goes wrong". There are freak pop-up thunder storms & not everyone has in-cockpit weather... $\endgroup$ – voretaq7 Dec 25 '13 at 23:50
  • $\begingroup$ @voretaq7: Haha, yes there are a LOT of people without in-cockpit weather. $\endgroup$ – Lnafziger Dec 25 '13 at 23:55
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    $\begingroup$ I thought the best altitude to be at in a thunderstorm was always zero feet. $\endgroup$ – DJClayworth Jan 2 '14 at 1:20
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The obvious answer you mentioned should be reiterated: Avoid thunderstorms whenever possible! Just because there are "best penetrating altitudes" doesn't change the fact that thunderstorms are extremely hazardous to aircraft and should be avoided in the first place.

With that said, the FAA's Advisory Circular 00-24C mentions that, if unable to avoid penetrating a thunderstorm:

To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15ºC.

This is to help minimize rapid accumulation of clear ice:

Supercooled water freezes on impact with an aircraft. Clear icing can occur at any altitude above the freezing level but at high levels, icing from smaller droplets may be rime or mixed rime and clear. The abundance of large, supercooled water droplets makes clear icing very rapid between 0ºC and -15ºC, and encounters can be frequent in a cluster of cells. Thunderstorm icing can be extremely hazardous.

Another point to mention is that, regardless of there being a recommended "penetration altitude," it is important to maintain a constant attitude, not altitude:

It is almost impossible to hold a constant altitude in a thunderstorm, and maneuvering in an attempt to do so greatly increases stress on the aircraft. Stresses are least if the aircraft is held in a constant attitude.

But again, emphasizing the dangers of thunderstorms, the circular states:

Never regard any thunderstorm lightly, even when radar observers report the echoes are of light intensity. Avoiding thunderstorms is the best policy.

and

Weather recognizable as a thunderstorm should be considered hazardous, as penetration of any thunderstorm can lead to an aircraft accident and fatalities to those on board.

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0 AGL. Do not ever penetrate a thunderstorm in an airplane. It is stupid. People die that way every year.

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  • $\begingroup$ I agree (and said add much in my question), however that isn't the answer that they are looking for here.... $\endgroup$ – Lnafziger Dec 25 '13 at 12:53
  • $\begingroup$ I know. I just wanted to emphasize this. Yours answers the question better. $\endgroup$ – xpda Dec 26 '13 at 6:44
  • $\begingroup$ Is that advice specifically for small planes & general aviation or for everyone? $\endgroup$ – curious_cat Jul 19 '15 at 19:33
  • $\begingroup$ Everyone, including airliners. Armored research planes may be an exception. ztresearch.wordpress.com/tag/t-28-storm-penetrating-aircraft $\endgroup$ – xpda Jul 20 '15 at 2:49
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I was asked this question on a 135 ride. Stumped for an answer, I was told I will get comfortable flying through if I wanted the job. As this was in the plains, 5000 or 6000 depending on the direction of the flight was the recipe. Not high enough for icing most of the time, yet enough altitude to recover from downdrafts. This also kept us in Approach/Departure airspace and their radar was much better at depicting live weather, as opposed to what the Center controllers handling higher altitudes would be seeing. Asking for vectors through level 1 or 2 was OK but taking level 3 when unavoidable was a rough ride. Using tango prefix in the call sign let them know I was neither a doctor nor an attorney. Night time lowered their workload so they could find a way to get us across squall lines, coming up with diversions around this stuff. After getting my head slammed into the ceiling I learned to hunch when it got bumpy. Few months of that, seeing thundersnow and learning that lightning flashes looks pink when you get close made me go back to flying passengers. Soon after the Check 21 legislation killed that whole business model but for many many years before me guys and gals were doing this every night with no accidents or incidents related to the practice.

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  • $\begingroup$ Great answer. I too did some work in the plains and across the rockies. The plains have, by far, the worst storms. Thunderstorms can be penetrated but it was always my last resort, and only with onboard radar. I would not attempt it with delayed weather services overlayed on EFB charts. $\endgroup$ – acpilot Jun 21 '18 at 23:44

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