# Is it difficult for a GA aircraft to escape a thunderstorm?

I live in the midwest, and I'm getting ready to start working on my PPL. For some reason I find myself really nervous about getting into the air and suddenly seeing a line of thunderstorms rolling in from the southwest. In my mind it feels like a fast moving front would be difficult to escape...

But I wonder if my fear is wholly unjustified. So I thought I'd ask:

If I'm in a GA plane (like a Cessna 172) and I see a thunderstorms forming to my southwest (lets say 20 miles away), how much time, in general, am I going to have to get on the ground before the storm gets to me? If I turn away from the storm, and start looking for an airport, will I be able to put distance between me and the storm to buy time to get on the ground?

Just to clarify before I get a lot of "that's what preflight briefings are for", this questions assumes there was a preflight briefing.

But thunderstorms have this tendency to show up unannounced where I live (or at least it feels that way to me), and that's what this question is about. What do I do when a thunderstorm, that I wasn't aware was going to be there, suddenly shows up on the horizon headed right for me?

• Your first line of defence would be to never find yourself in such a situation. That's what pre-flight briefings are for. – J. Hougaard Aug 16 '16 at 14:59
• Some isolated CB's might appear "out of nowhere" during days with high convective activity and an instable atmosphere. Just fly around them. It's also pretty easy to estimate the likelihood of this by conducting a proper weather briefing (a diagram of the temperature gradient is a good place to start). Fast moving fronts and squall lines do not just appear "out of nowhere". Have a look at sigwx and frontal charts before you go flying. – J. Hougaard Aug 16 '16 at 15:05
• The Cessna 172R is given as having a cruise speed of 122 knots, and a $v_{NE}$ of 163 knots. (Check the specifications for the model you have in mind.) 163 knots is 302 km/h is 83.9 m/s is 275 ft/s. What wind speeds do you anticipate at the cloud layer altitude? Unless you expect winds aloft to be at least twice hurricane speed and for these to be maximally unfavorable for you, then mathematically, you should be able to outrun even an unforeseen thunderstorm system that you see forming. – a CVn Aug 16 '16 at 15:08
• @MichaelKjörling In addition, if the wind is moving a CB toward you at 50 knots, then it is also moving your plane in the same direction (away from the CB) at 50 knots. Planes are subject to wind just like clouds are – J. Hougaard Aug 16 '16 at 15:09
• @CodyP The questions is about what to do to avoid flying into a thunderstorm (it's why I mentioned the storm being 20 miles away.) What happens if you end up in a thunderstorm cloud is this: "Wind sheer tears your tiny little GA craft into small pieces and you die a terrible death." That's why I'm asking about avoiding thunderstorms in a GA craft. (source: nydailynews.com/life-style/…) – Jay Carr Aug 16 '16 at 17:32

If you take off with a storm 20 minutes away from the airport, it won't have been "unforecast"; the NWS is much better than that. They can pretty accurately predict what's going to happen regarding various fronts at least 24 hours in advance, in turn giving you the advance notice to adjust your flight schedule to ensure you can get around the storm or well away from it.

But let's say someone goofed (probably you), and now you find yourself climbing into the air off the tarmac with the leading wall of a cumulonimbus looming over the horizon, which will be between you and your intended destination before you reach it.

First rule; never fly toward the storm. Cumulonimbus formations are deadly to small craft, and even commercial airliners prefer to stay at altitude in a holding pattern well above the storm and let it pass by before attempting to land. These "thunderheads" produce torrential rains, powerful and frequent lightning, strong updrafts, hailstones ranging from pebbles up to grapefruit, and turbulent cyclonic winds that can culminate in tornadoes. Your Cessna 172 stands no chance.

Flying away from the storm will, at worst, gain you the time between the storm's speed and yours that you can use to find an open airstrip and put down. As Michael Kjorling said in his answer, most Cessna 172 variants cruise at over 100kts, which would require the storm moving across the ground as fast as a Category 2 hurricane rotates, so in most scenarios even if you're flying in still air with the storm moving over the cooler air you're flying in, if you just put the squall line to your rudder you should gain ground. Ideally, the storm will be pushing a mass of air in front of it which will give you a tailwind, and thus you'll be able to put that much more distance between you and the storm front.

Second rule; unless your planned flight is a cross-country flight heading pretty directly away from the incoming front, get on the ground as quickly as you can do so safely, and find a shelter for your aircraft. Obviously, being at your home FBO where you rent your tiedown or hangar is best, but anywhere you can slip your aircraft under a roof or awning for a couple hours will do. Remember that unless you have your instrument rating, the conditions during and after most thunderstorms are IMC and thus illegal (and deadly) for you to fly in.

If you're flying a pressurized aircraft, and/or you have instrument privileges, you have more options, the best of which is to get above the storm and wait it out. However, your Cessna 172 is not pressurized, so unless you have an oxygen bottle aboard (and are endorsed to use it) you're limited to about 12,500 feet and only for a half-hour, well below the top layers of a thunderhead, so you'll have to outrun it, get around it, or put down and shelter in place while it blows over you.

• The NWS most certainly cannot predict the position of a pop-up CB 24 hours out. They can tell you the probability of a CB passing over a given position within a several-hour period that far out, but they have no idea where the actual individual CBs will form until they start forming. Of course, at 20 miles out, you can simply look up and you'll see it. Those things are huge and can be seen from many, many miles away. Once you're airborne, you can see the towers from over 100 miles away, provided there aren't other clouds blocking your line-of-sight. – reirab Aug 17 '16 at 5:07
• "the best of which is to get above the storm and wait it out" Unless you have an aircraft with a ceiling similar to that of an SR-71, this is a really bad idea. CBs can go over 50,000 feet and commonly go over 40,000. Also, conditions after a CB passes aren't necessarily IMC. During obviously is, but the IMC is probably the least of your concerns at that point. – reirab Aug 17 '16 at 5:28

Practically speaking this isn't much of an issue, at least under VFR and if you're paying attention. Thunderstorms take some time to form, and as long as you can see them you can avoid them.

I'm assuming you're asking here about isolated thunderstorms, the kind that form in late afternoon in hot, muggy conditions and aren't part of a frontal system. Typically - at least in places like AL, TN, KY etc. - you get a widespread broken cumulus layer, with isolated thunderstorms building up in spots where conditions are more favourable. Something like this:

By Jessie Eastland (Own work) CC BY-SA 4.0, via Wikimedia Commons

Whether you're above or below the broken layer, a storm like that is usually very visible and you can easily fly around it. Above the layer you can see the towering cumulus; below it you can see the rain and probably also the build-up, through gaps in the clouds. You might also have ADS-B or another weather source in the cockpit but the data is already a few minutes old when you get it. It can still be a good way to get a picture of the overall direction that the storm's moving in and whether it's growing or not, but it's definitely not something you can use for real-time flying around a storm; your eyeballs are best for that.

When you see a build-up or an actual storm, you need to assess how far away it is, how severe it is, and which direction it's moving in. You can usually do that visually, but cockpit data can help. Then, simply plan to fly around it. You mentioned 20 miles, which is the distance the FAA recommends to keep between you and a thunderstorm, to avoid turbulence, hail and other unpleasantness (like your passengers getting sick). If you decide that you can't fly around it safely, considering the overall weather situation, terrain, fuel etc., then just land somewhere and wait it out.

But do make a clear decision on what heading to fly; never keep flying towards a storm or large build-up to 'see how it looks'. If your new heading isn't working out the way you want, then aim further away from the storm or turn away completely. A VFR cross-country flight in the South in summer often ends up being a series of extended zig-zags to work around the build-ups. Personally I think that if you're on top of the broken layer it's beautiful and and watching the towers build up is fascinating, but don't get too close! And the towers do build up over time, a thunderstorm - even a small one - is far too massive to just appear out of nowhere, so you really have to be doing something wrong to get caught close to one, at least in the weather conditions I'm describing here.

Under IFR, things are a little different because you have a specific route to follow but in the end it's surprisingly similar to a VFR flight. In summer you'll hear IFR pilots at all altitudes and in all types of aircraft (including airliners) asking ATC for a deviation:

N12345: Center, N12345 requesting deviation 10 degrees left for a [cumulus] build-up
Center: N12345, deviation approved as requested, report direct ABC VOR when able

That's exactly what the VFR pilots are doing too, you just need ATC's permission for the heading changes under IFR. And I made some other comments about VFR/IFR and storms in this answer; they might be relevant here.

• Your assumption in the second paragraph is the case the question is explicitly not asking about: it refers to a "line of thunderstorms" in the midwest, not isolated thunderstorms in Alabama, Tennessee or Kentucky. – David Richerby Aug 17 '16 at 16:04
• @DavidRicherby If you look at the edit history and the comments, you'll see that originally the question asked about "a line" of unforecast thunderstorms, but the OP removed it because he agreed that it's hard to miss a squall line in a briefing. My understanding is that he is indeed asking about isolated or 'pop-up' thunderstorms, which are very common in the South and many other places. – Pondlife Aug 17 '16 at 16:12
• OK but I'm a bit confused. The edit history says that the question hasn't changed since half an hour before you posted your answer. (I checked for that before I commented, since "the question changed after I answered" is a common reason for this kind of mismatch.) – David Richerby Aug 17 '16 at 16:15
• @DavidRicherby You can see it in revision 3 from yesterday, and this comment – Pondlife Aug 17 '16 at 16:17

First thing first, although it is already mentioned, weather forecast is covered in the briefings. If the atmosphere favors thunderstorm formation, there are signs to look out for.

Now, thunderstorm clouds don't form out of nothing - meaning like, if you got "CAVOK", unless you do your takeoff 4 hours later, chances of you encountering a thunderstorm is very slim. Pre-thunderstorm clouds are usually observable 1~2 hours before the you see rain and lightning. Many people do not realize this simply because they seldom look up the sky. Signs will include low altitude cloud covers, fast changing cloud shapes etc. You will learn in your ground school how to name and identify clouds.

Thunderstorms that form quickly also dissipate quickly. In tropical areas, e.g. Singapore, it is pretty common to have a sunny morning followed by rainy afternoon. However, the area of the thunderstorm is quite small. They are not like massive systems which stretch across states. You should be able to outrun the storm or wait for it to move away without running out of fuel. Often, there will be holes around 10~20 miles wide between storms.

One thing to watch out for is small-area storms with intensity. One of the AOPA lectures mentioned this as the "weather gradient": unlike massive weather systems, where the change from light chop to moderate to heavy turbulence is a long distance, you get from smooth air to turbulence air in a very short time when you encounter these type of clouds. The bad thing is things go bad very quickly. The good thing is safety is often not far away. Always know your heading, and practice making a 180 turn with instruments alone.

If you need to get down, be sure to know where are your alternate airports all the time. It goes without saying the same applies to other emergencies as well. If you have a rough engine or you start feeling dizzy, you better be turning your plane towards the runway instead of scanning your aerial chart nervously looking for the airport symbol.

Lastly, learn how to obtain weather updates while en-route. Your Cessna 172 likely would not have a weather radar, but there are other ways to keep yourself updated - satellite links, radios, etc. If you tune yourself to the local flight-following frequency, you can hear pilot reports from planes ahead of you (you can just listen and not use flight-following). Or you can try the AWOS station 30 miles away. If you're using a satellite link for a radar image, be aware that the images are usually 5~15 minutes old. Never try to navigate through a storm with satellite images. The storm will have moved by the time you think you're in the clear hole. However you can use it to spot rain clouds 80 or 100 miles away.

Side story: there is a story circulating the local flying club from around the 80s or 90s. At that time, GA planes share the only airport, which has only one runway, with commercial airliners. To keep things speeded up, GA planes which depart in the afternoon are not allowed to come back until 2~3 hours later (unless it's an emergency), since sequencing a C172 between two B747s is a nightmare when the airport is operating near full capacity.

So, one GA pilot departed on an afternoon, and he really didn't like weather on his climb out - clouds were nearly overcast behind a mountain, and 2000 ~ 3000 feet was nearly IMC. He radioed in asking if he can land. The controller asked, "How is the weather out there?" The pilot was so nervous and so worried about the weather, he keyed his radio and transmitted:

1) Preflight planning - as mentioned above modern meteorology can quite accurately predict where convective activity will be at least 24 hours in advance of your flight so you will know if there is a high risk of thunderstorms along your flight path.

2) Obtain a standard weather briefing prior to your flight. The briefer can give you area and terminal forecasts at your departure and destination, applicable AIRMETS, SIGMETS and Convective SIGMETS along your route.

3) Local conditions - remember, thunderstorms require three ingredients to form: unstable air, moisture and an orthographic lifting factor. If any of those items are not present, a thunderstorm will not form.

4) Don't let yourself be painted into a corner. This is good advice for any flight planning; always have a backup plan - and backups to the backup plan. Carry extra fuel. Andicipate alternate or divert routes should adverse weather form along the way. The good news in the Midwest is that you can practically open any place on any char and find an airport nearby. If you are flying with a GNSS navigation system the NRST function will give you the name and heading to any public airport Nearest to your current position.

5) Climate factors - the good news is in the Miswest thunderstorms form along frontal easily anticipated frontal boundaries. Summertime in Florida is another matter where airmass thunderstorms pop up all over the place and develop very rapidly. The bad news is that Midwest squall lines are some of the most powerful storms on the planet next to cyclonic storms, so stay well away from them.

6) Making use of NEXTRAD and satellite weather products in route. These have become excellent resources for pilots. Be aware however, there is an much as a 15-minute delay in the NEXTRAD picture; NEVER rely on it as an instantaneous picture of the weather environment but use it instead as a trend information source - to indicate where storms are moving or developing in. Remain at least 20nm clear of yellow or red cells or other areas which indicate a storm may be present.

Going by the figures given by Wikipedia, the Cessna 172R has:

• Cruise speed 122 knots
• $v_{NE}$ (never exceed) 163 knots

I also checked the corresponding numbers for my club's Cessna 172N in its POH, which gives $v_{NO}$ as 128 KIAS (126 KCAS) and $v_{NE}$ as 160 KIAS (158 KCAS). For the purposes of this answer those are close enough, as well as slightly faster (so using the figures from Wikipedia means we're wrong in the direction of safety). For completeness' sake, this aircraft's POH gives $v_{A}$ as 97 KIAS for 2300 lbs, dropping practically linearly to 80 KIAS for 1600 lbs, with KCAS just a hair below (same or one knot slower).

Obviously, you'd want to look up the numbers for the model airframe you have in mind, but the above should be good for a starting point.

Because you really want to get away from that storm system, let's assume you are doing your best to move away from it. I'll consider three scenarios: at maneuvering speed (calling it 90 KIAS); at cruise speed (122 KIAS); and at just below $v_{NE}$ (calling it 160 KIAS).

90 knots is 167 km/h, or 46.3 m/s.

122 knots is 226 km/h, or 62.7 m/s.

160 knots is 296 km/h, or 82.2 m/s.

An all-out hurricane is considered to begin at wind speeds around 30 m/s.

If the winds at the cloud layer altitude is pushing the clouds toward you, and you are maximally unfortunate (equally strong winds in the opposite direction, directly from the direction you want to go; I don't know how that would be possible at any reasonable altitude), then:

• At maneuvering speed, the storm will be approaching you at about 16 m/s
• At maximum cruise speed, you will be keeping pace with the storm system, but not really getting away from it
• At near $v_{NE}$ in something resembling level flight, you will be moving away from the storm system at about 20 m/s

A more likely scenario is that the wind direction is similar at your altitude as that of the storm clouds, which means the wind will be pushing you and the clouds equally. This causes your speed relative to the clouds to be your indicated air speed reduced by the wind speed, which at cruise speed and hurricane winds means you are moving away from the storm system at a significant speed; even at 90 KIAS, you're still moving away from it at about 16 m/s.

Thus, if you see a storm system forming directly ahead, and you make an about-face turn, mathematically, you should be able to outrun the storm system just fine without over-stressing the airframe; likely without anywhere near over-stressing the airframe. As long as the storm system doesn't form practically on top of you, you can most likely just leisurely cruise away from the storm system and land at the nearest appropriate airfield with no significant worries. As long as you head for calm conditions, even a totally unforeseen storm developing (which, as has been pointed out, is highly unlikely in practice) shouldn't be that difficult.

A suitable fuel reserve and a cool head would appear to be sufficient for getting back down on the ground relatively safely, if perhaps not in the exact location you intended when taking off. Some practice operating the aircraft in turbulent conditions might help.

• I appreciate any feedback on any relevant aspect I might have overlooked. I'm not a pilot, I mostly just play one on the computer from time to time. – a CVn Aug 16 '16 at 15:25
• Unless you are in a dive, hitting anywhere close to Vne in the 172 is going to be difficult. Even at full throttle in level flight you aren't going to hit that speed. Not that it really makes a difference, even at cruise speed you are probably going to outrun it. The issue is that convective currents from thunderstorms often extend out 20+ miles from the cloud bases. This is most dangerous in the landing phase, although turbulence can rip the aircraft apart before you even get close. – Ron Beyer Aug 16 '16 at 15:27
• @RonBeyer Fair point. How do you feel about it now? – a CVn Aug 16 '16 at 15:49
• @MichaelKjörling I'd use a speed of 130kt IAS, rather than Vne. 130 kt is roughly Vno (Maximum structural cruising speed). Vne is for smooth air, which you're unlikely to encounter near a thunderstorm even when running away: Vno is the non "smooth air" equivalent, and 130 kt also more of an achievable airspeed in level flight. You'd also be recommended to stick to Va (manoeuvring speed) if things get rough, which is more like 100 kt. Using 100kt and 130kt for your "worst/best case" speeds would suit the C172 better – Jon Story Aug 16 '16 at 16:00
• If you're going to be doing the math, you will want to factor in how much time it takes to actually line up at a runway, slow down for landing, get on the ground, taxi and tie down before the storm hits. Keeping in mind that fuel may be limited as well. – Jay Carr Aug 16 '16 at 16:05

There's a lot of detail above but rather than repeat, I add there is a big difference between a FRONTAL thunderstorm line and isolated thunderstorms (that are common in the midwest and rare at the code). Isolated thunderstorms are relatively easy to avoid by going around them. Frontal lines are difficult.

I also clarify that airliners do not generally go above thunderstorms. Thunderstorms usually rise to over 50,000 (or higher) and they can grow very quickly.