IFR flying, new IR rated, cumulus clouds
 

Hi all aviation people.
Wish you à nice and good summer in the skies.

I am not totally new to flying but quite new to IFR flying. A year or so. I thought i share some experience with some questions for you more experienced IFR pilots.

I fly in Europe and mostly with C172. Recently i happened to end up in à pretty intense shower during one of my flights. During that period i observed that the vsi instantly showed 500 ft decrease and i lost 100 ft instantly. NeXT second i could get increase the same way.

But the funny was that nothing felt in the airplane. What happened really? Have not seen this before. Was it very fast pressure changes? Very strong up and downdrafts? Were i in some dangerous situation? Accidently in CB? Have you experienced the same?

This discussion brings me to next topic. How big cumulus clouds is safe for à small aircraft to penetrate? Of course CBs should be avoided, but how do you make your decisions for other cumulus clouds? Towering ones?

Thanks à lot, your experience about above topics would be very much appreciated!! :)
Take care people.

Hello Krallu

Sounds like you flew through some up- and downdrafts. Quite normal and not dangerous if you're flying within limitations. Even fairly small cu can give you quite a bumpy ride in a 172. Would certainly avoid anything big, dark, towering or not isolated.

What you flew through was not a CB.

Enjoy your flying, stunning up in your part of the world. :)

Regards

The definition of a CB has been discussed at length on PPrune, for example here.

If is was a shower, the existence of precipitation implies glaciation at the top of the cloud, which is probably enough to call it "cumulonimbus".

In practice, there's a continuous spectrum of cloud from little fair weather cumulus to the monstrous tropical cumulonimbus the everyone wants to be at least 50 miles from. The working definition among pilots of the difference between "cumulus" and "cumulonimbus" is that the line tends to be drawn by any given pilot at the size of cloud that she is not prepared to fly through.

Male pilots on the other hand always fly through them!

In the UK, we glider pilots use the updrafts (“thermals”) that form cumulus clouds. They vary in strength from fractions of a knot to 10 knots or more (1000 feet per minute) vertically.

Air going up has to be replaced by sinking air somewhere else, sometimes quite close to cu. Downdrafts can be as vigorous as the updrafts in the same area, but in my experience are usually wider and thus less strong.

Many continental countries have stronger thermals. I have no personal experience of Scandinavia, but there are very good gliding conditions there in summer, judging by reports I have read.

Chris N.

Chris

Thermals are only one of the three ways that Cu is produced, a thermal is a convection up draught.

Downdraughts only exist in the mature and subsiding stage of the cloud, in the initial buildling stage the cloud is composed of updraughts which may be caused by convection, orographic or frontal lifting.

These statements do not compute.

Hmmm, true, he did say he felt nothing, missed that... dodgy instruments?

Is it possible to get rapid pressure changes enough to affect an instrument but short lived enough to not produce sudden lift/sink (inertia)?

Had that on very sensitive VSI's, yes, but not on altimeters. Anybody?

I'm not sure if this explains all the symptoms, but could one factor be water droplets from the shower blocking and then unblocking the static port? I mean covering then uncovering the port, rather than going up it.

The aircraft might have been in a gentle climb due to a mild updraft.

This might not be noticed in the poor visibility of the intense shower.

Each time the port was covered then uncovered:

• The altimeter would freeze at a historic height, then unfreeze and suddenly show the true height.
  
• The VSI would slowly fall to zero, then suddenly show a large erroneous climb rate. (I think).

Having seen how an ASI needle flicks about as a bug gets pushed up the pitot, I can definitely believe the 'instant' movements.

Edited to add: it might be streams of water rather than droplets.

Ok thx. Interesting posts.
I cannot with 100 % certainty say if i did gain and loose height for real or not. 100feet is not much. But it sure felt strange because i remember noticing the very instant altitude readings and i was thinking at the moment, shouldnt i feel this.

The instruments else works fine and exact.

The water blocking is one theory because it was some heavy rain indeed.

Interesting.

Anyhow no matter what actually happened or what size is defined as CB or not i am more intersted in how you decide what weather to go imc in or not. The hardest one for me is when the weather reports cumulus and some you can see in the afternoon starts to build, and one or more occasional showers. But no reported CBs yet. Do you go imc in that weather?

If there is isolated CBs do you go imc and then around the CBs if you see them?

I guess embedded CBs you avoid and go visual?

Thanks! :)

Something possibly missing from my IMCr course was additional weather training, along the lines of "see that cloud? the reason you can/can't fly through it is x,y,z" (apart from icing considerations).

''The hardest one for me is when the weather reports cumulus and some you can see in the afternoon starts to build, and one or more occasional showers. But no reported CBs yet. Do you go imc in that weather?''

Probably, but depends on the route, availability of alternates, availability of regular weather updates, terrain, duration of planned flight, equipment, endurance, experience level, etc, etc. But there's a lot you can do. Select a level that puts you in VMC whilst on an IFR flight-plan, so that you can avoid visually if necessary. Get reports from aircraft ahead. Get reports from ground stations ahead. Ask for avoidance around build-up. Enter a hold and wait it out, the fireworks can be over quickly, or it may move along with the wind. A quick diversion to wait for 30 minutes before resuming your trip is no big deal, as is a smart 180-degree-turn, and so on. But in the end, if I ascertained that there was any chance whatsoever of accidentally penetrating a CB (the real deal, not the fluffy little Cu that's a bit moist and choppy, you know what I mean), I wouldn't go.

''If there is isolated CBs do you go imc and then around the CBs if you see them?''

No, if I did not have the means to avoid the CBs (i.e., radar) I would not go IMC, too great a chance of accidentally penetrating one.

''I guess embedded CBs you avoid and go visual?''

If there were embedded CBs and I had no way of detecting them if in IMC, I would definately remain VFR.

As you say you felt no A/C movement
You might have observed the effect of you static vent becoming covered by rain water, as the rain flowed over the static vent.
On modern C150/2 C172 the static vent has round washer shape that extend 1mm above the fuselage skin, on older 172 this is not the case and the static vent is flush with the skin.
In very heavy rain it flows over the vent and stops static pressure changing. so your VSI will go to zero and your Alt will stay at the last setting that it had static pressure
This effect nearly killed me 20 years ago when descending at night IMC in heavy rain in a very old C172 and I came visual at 300 feet with Alt still showing 1700 feet that then jumped to my actual altitude.
Suggest you have your static system checked, additionally your A/C might have an alternate static source that allows cockpit static to the ALT/VSI.

Krallu

If you flew into a cumulus cloud its likely you did experience up and down draughts.
Check if your VSI and altimeter are heading in the same direction! cross check that with airspeed.
Rising air airpseed will increase as you are trying to fly level.
Falling air and your airpseed will decrease.
When flying PAX we try and avoid them as they are bumpy If you cannot avoid be sure of where you are heading especially IMC with the embedded variety.
Without radar all you have are your eyeballs to read whats ahead and your best way through and that usually takes some experience.
Look up towards the tops and try and pick the valley sections.
If the bases are above the SSA go below and steer around the rain shafts.
Beware of the Outside temp as the temp will drop in the clouds which could lead to being in icing conditions.
If you experience turbulence reduce speed and dont fight the aircraft (it flies better than you :)
There is always an arguement whether to do a 180 or carry on.
If you do a 180 make it gradual.
But really check the met for your route and especially the rainfall radar you dont really want to be messing with anything larger than cumulus or towering CU.

Pace

staying in VMC
 

this is what we did this week somewhere over Germany - steering to stay in VMC - it doens´t look we are going to make it due to A/P minioscilation, but we stayed just between two tops....
Lelystad 2011

Pull what said:
I regret that your statement that downdrafts only exist in the mature and subsiding stage of the cloud does not fit with my experiences over many years of gliding. Taking thermals, there is sink (downdraft) all around them virtually always - even before they have risen to a height where condensation occurs and cloud forms. When the cloud forms the thermal rises more rapidly (wet adiabatic lapse rates & all that) and the surrounding sink does become stronger. At a later stage when precipitation has occurred the downdrafts increase in strength and are usually strongest on the upwind side of the cloud.

A risk in flying in or near very large cu / cunims is that they can grow in size much faster than you might have been expecting and what you thought might be 5 minutes in IMC could end up as 20/30. They can also get very rough making handling difficult - even ignoring the fact that you are on instruments.

Thanks for great posts!

Just another quick question for you. How do you log the instrument flight time. My idea is that you only log that when you fly IFR and you fly solely on instruments.

For example a flight IFR in CAVOK: You can't log any instrument flight time.

An IFR flight in IMC: You can log instrument flight time.

IFR flight in partial IMC: You estimate the time on instruments.

Is this correct?
Thx!

First, an answer to the original question - I see the same behaviour all the time flying a C172 in the rain. As Orionsbelt described, the basic 172 has only single static port, and it is susceptible to water droplets running across it. If you see flicks to +/- 200 fpm and the altimeter jumps up or down 100 feet without there being a corresponding physical "bump", that is what you are seeing.

Answer to the second question, you should log all the time that you fly by reference to the instruments, regardless of the flight conditions. If you are IFR in controlled airspace, even if it is a CAVOK day, you should generally be flying on the instruments because your "VFR" skills and altitude/heading keeping will probably not be to IFR standards. Hence, broadly speaking if you were flying IFR, log it as instrument time.