Can anyone explain this?

[JamesG]

Firstly. may I state in advance that I am not an instructor. I know that this forum is not for PPLs to ask for advice, but I hope that you will excuse my posting - perhaps even find it interesting. I just do not know if any other forum would have enough people with the experience in light aircraft to consider this question.

Back in the early 90s when I was a student pilot (about 30 hours total) the flying club organised a fly out to France. I was keen to go and my instructor felt that it would be a good opportunity to do something different.

The weather on the June day saw a warm front lying over the route, with layer cloud forecast from 800 feet to an altitude above the a/c service ceiling, with light rain, zero to light icing etc. Light winds.

My instructor(frozen ATPL) offered me the option of a no go, but said if we did, it would be a very good opportunity to handle the a/c in real IMC and see just how challenging the task was. So off we went.

The outbound leg went very smoothly and so did the return (my instructor checked the weather and it was similar to the o/b forecast)

About 20NM south west of the Dover VOR, some strange things happened very quickly:

1 The groundspeed on the DME went from circa 100kts to 45kts in a few seconds - the airspeed never flickered.

2 Then the airspeed indicator needle fell back against the stops and I would have suspected catastrophic failure if the right wing hadn't dropped as we stalled.

3 My instructor took control instantly and at that point we ran in to very heavy rain - it went very dark, but there was no hail or airframe icing.

4 The VSI then went smoothly against the "up" stop and the altimeter started to increase rapidly.

5 My instructor applied full carb heat and closed the throttle - less noise, no change in the VSI or altimeter.

6 We had another couple of stalls/wing drops and the ASI was all over the place.

7 Strangely enough, the air was not really bumpy, I have rougher rides under CAVOK skies with a bit of wind about.

8 The throttle was then opened fully and the nose pitched down. We were still climbing.

9 Suddenly, we hit a small bump and the VSI and alitmeter reversed. The throttle had to be closed to restrain rapid ASI increase.

10 After about two minutes, we exited the cloud base (layered) indicating 700 feet QNH. On checking in to Manston as we were coming to mid-channel, we were given a QNH which reduced the altimeter reducing to 500 feet.

We arrived back safely and discussed the matter over a couple of pints with the CFI. He had never experienced anything like this himslef and his best guess was that the warm front had produced enough of a lifting effect to spawn a CB, which we had been unlucky enough to get rather too close to.

We did a "forensic" check of the actuals both sides of the channel, but never found any trace of convective activity.

If anyone has an opinion as to what happened and the cause, I would be very grateful as this experience has confused me ever since.

Thanks





[This message has been edited by JamesG (edited 16 November 2000).]

[WeeWillyWinky]

Can't explain what you experienced but sounds as if you were very lucky. I have a slightly different view of IMC flying in a single since suffering a catastrophic engine failure when in cloud. I also have a pal that narrowly survived a ditching in the English Channel also after an engine failure. I am lucky enough to fly jet aircraft now and although I still fly light aircraft I am ultra cautious about the weather and especially if going over water.

[This message has been edited by WeeWillyWinky (edited 16 November 2000).]

[JamesG]

WWW

Thanks for your comments. I must admit that the experience has put me off flying in IMC and I do a couple of hours under the hood every year just to retain the ability to fly a controlled 180 and get out of Dodge.

Perhaps more importantly, like you I am ultra cautious about the weather and you are not the first jet pilot to counsel me on the dangers of light singles and IMC, advice which I take very seriously.

[Tinstaafl]

Sounds a bit like a microburst or shear between strong up & down drafts.

I got had an 'interesting' experience in a developing TS in a Baron once. Cruising at 8000' then there was an express elevator ride to 12,000. Even with power reduced to 15" MAP. Not much turbulence apart from typical Cu related.

Otherwise I'd think icing with its increased Vs etc.



[This message has been edited by Tinstaafl (edited 16 November 2000).]

[Noggin]

This sounds remarkably like a blockage probably by ice, of the pitot static system, the rest was probably self induced.

[DB6]

JamesG, don't know what may have happened in the air but as to the first bit of your post, as an instructor myself I had assumed that this forum is exactly for PPLs asking for advice, amongst other things. That's what we're here for ! Hope I'm not speaking out of turn and if other instructors disagree I'm sure we'll soon know about it.
Cheers DB6

[Wee Weasley Welshman]

JamesG - very odd. First thoughts are pitot/static iced up?!!?

You are welcome to air such instructional stories and queries on this forum. Do any of you istructors visit the Private Flying forum... you should.

WWW

[Tinstaafl]

I had thought he said heat was on?

reads it again...

oh...carby heat, not pitot heat. http://www.pprune.org/ubb/NonCGI/redface.gif

[JamesG]

Thanks for the replies. It has certainly given me some food for thought.

I can confirm that the pitot heat was tested pre-flight and selected "on" before departure ... but I suppose that is no guarantee that it was functioning at the time of the occurence. I don't recall checking the circuit breakers during or afterwards - I don't know if my instructor did - we were both pretty shaken.

There was no visible airframe ice and the OAT was above freezing.

The idea of self induced problems had not crossed my mind, but thinking of experiences in my day job (as a consultant/programme manager of complex projects) problems are often caused by human error (usually a number of small, yet critical, oversights or porr decisions.)

It is human nature to be defensive and look for external causes, when in fact the reality is closer to home.

The counter balance to this view is that it was a training flight and I had an instructor fresh from an IR monitoring me closely from the RHS - of course, it could have been a joint screw up!

Finally, the ASI reading zero was transient. I am note sure how long the incident lasted for, but it could not have been too long, as we "only" gained about 1100' with VSI against the up stop. As soon as we hit the little bump and the VSI reversed, the ASI shot up from zero to over 100KTS in seconds. The a/c was a PA28-140 by the way, don't know if this is significant.

[Noggin]

The Pitot and static are on the same mast under the wing, it has two heating elements which are prone to failure if left on, on the ground. You can tell from the ammeter if one or both are working. Rapidly changing pressure instruments immediately raises the question of a blockage, transient or otherwise. Stick to basics, Attitude + power = performance, leave the throttle alone, maintain a level attitude with wings level using the gyro instruments. Chasing what were probably spurious indications may have resulted in the situation you describe. It is interesting that we practice limited panel simulating the failure of gyro instruments but not the failure of pressure instruments. Food for thought.

[Skycop]

I agree that it does sound like a micro-burst although these are not normally associated with the UK climate.

I believe I suffered one in Central America about twenty years ago, flying an RAF Puma helicopter with 14 POB. As we entered a rain shower (not at all unusual out there in season), the aircraft was basically propelled downwards against full power. There was no turbulence as I remember, just a powerful downwards surge as the rain intensity suddenly increased markedly and we lost visual references.

As full power and a flare from 120 kts to 70kts (Vy) had little effect in getting the VSI needle off the bottom stop, for a few seconds I had the sudden thought that both engines had flamed out (which of course they had not or I would have had no rotor rpm worth talking about with max. collective pitch applied).

I carried out a steep turn through 180 degrees and a few seconds later, as we came out of the rain, the aircraft suddenly climbed like a cork out of a champagne bottle. The event began at about 1500 ft and we recovered to the climb at about 200 ft above the jungle. I had put the gear down as I was convinced we were going in! I went straight back to the departure helipad a chastened man and we stayed there overnight. No-one argued, especially the crewman.

I too could not understand what had happened and no-one I spoke to had experienced anything like it. I later discovered an American paper on micro-bursts. Basically, mass ascent of air over a wide area can sometimes result in an extremely violent downdraught in the vicinity of Cu-Nim clouds. This is usually associated with an area of heavy rain.

Perhaps you experienced a similar thing, albeit rare for UK due to the lower i.e. non-tropical temperatures.

Was the air unusually unstable for the UK?

[Soaring Sprog]

Like JamesG I hope you don't mind my presence here- I am a low hours PPL, but have a few hundred hours gliding and found this thread quite interesting.

I spend quite a bit of time in gliders each summer inside towering cu's and the like (yes intentionally and for fun!) soaring the updrafts sometimes to quite suprising heights. The effects JamesG describes, to me sound perfectly indicitive of a powerfull convective updraft. In a glider, as I'm sure many of you know, the ideal when soaring in cloud is to circle in the core of the updraft. In my experience this core is usually exeptionally smooth, (and sometimes exeptionally strong!) It all gets a bit more interesting if your circle strays from the core. The boundary between the rapidly rising and equally rapidly descending airmasses can be quite turbulent to say the least. Holding a steady attitude, the airspeed and vertical speed fluctations as you cross this area of shear can be pretty amazing. Usually it's just a case of riding it out with steady, smooth IF. In a glider it's no big deal because you are flying WITH the system of the cell, not trying to fight your way through it at a nice steady speed and height as in a powered aircraft.

I would imagine that unitentionally encountering such conditions in a powered aircraft would be a whole lot less enjoyable!!

That's my perspective on it anyway!

[RVR800]

I met a guy who got a blocked
pitot on an initial IR check ride
509 er

He misinterpreted and applied control
column movement forward (to maintain
IAS)

The FCL guy had to take over control

It was a transient block air and speed returned to normal shortly after

[chicken6]

Yikes!!

Another theory for the cauldron...

As you were flying along, you were caught in entrainment (air being pulled in from outside) of a CB or TCU-related updraught. However, this updraught was centered off to one side of your track, and as it moves you sideways relative to the DME, this causes the drop in DME-related groundspeed (?)

Other ice-related theories then fit in

As the air you are flying in is now moving in the same direction as yourself, your airspeed now decreases

then the updraughts, downdraughts somewhere else in this mystery-cloud

when you popped out the bottom, can you remember if you were still on track, or had it moved you off to one side?

Personally, and with hindsight, I think the rapid rise in airspeed after encountering the downside may have been due to chasing the instruments, although if my airspeed suddenly went to zero I'd poke the nose down as well!!

Good story, well organised thoughts.



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Confident, cocky, lazy, dead.

[JamesG]

Skycop/RVR800/Soaring Sprog/Chicken6

Many thanks for your interesting observations.

To answer specific points:

Skycop - the airmass was stable, but there was heavy rain about in some areas (NS.)

Chicken 6 - we entered tracking towards Dover and exited roughly pointing towards Ostend.

Skycop - I empathise with you about the plummeting feeling with the VSI against the bottom stop - At least we were in IMC and couldn't see the ground rush!

RVR800 - interesting that you example demonstrates a transient blockage and that it was the FCL guy who diagnosed it. In our case, with a student PPL, there would have been no sanity check for the IR pilot who had to take over in very difficult circumstances, i.e. sitting in RHS with parallax view of instruments.

[This message has been edited by JamesG (edited 21 November 2000).]

[Skycop]

Nimbostratus! Yes. Possibly it WAS caused by a large embedded CB then. The area of heavy rain seems to act as the "plughole" for very rapidly falling air to replace the general, mass ascent of air.

[Tail Heavy]

Know the feeling! Some years ago, living and flying in Fiji.
Was flying at 7500 ft in an islander, luckily was a ferry flight only myself on board. Flying through a TS, went up, and up and up. Finally It stopped, at 18,000ft!!!

My advice to anyone out there, esp young guys dont get in an aeroplane with just anyone! I wont get into an a/c without a set of controls in front of me!

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The only time you have too much fuel is when you are on fire!!

[offshoreigor]

JamesG:

It would seem you passed through the frontal system. This happens quite often off the East Coast of Canada. The route from Halifax to Sable Island is about 140 NM. In that relatively short distance, you can cross through several developing systems due to the relatively warm water temp (gulf stream affect) and the cooler temps aloft. Even in the dead of winter you can run into a developing CB.

From the sounds of it, your pitot heat was fully functional and your instruments indicating correctly. The fact that you were tracking to a VOR and experienced a sudden decrease in G/S would confirm it. Especially since it was followed by rapid fluctuations in IAS. The VSI was pegging full up and down, this would definately support convective activity.

You don't have to be in a CB to get this, it could be a TCU or ACC. I have flown for many years (20+) offshore and am still surprised by the B@lls that Mother Nature displays.

This episode should not discourage you from IMC flying, on the contrary, it should inspire you to practice it even further to the point that you will be able to anticipate what The Old Girl has to throw at you and learn to respect your own ability.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[awilkes]

James
Think I know how you felt. Had a very similar experience to that in a C152 in Florida last year, flying as PIC with 80hrs TT !!
Got caught under rapidly developing CB's heading west to the coast from Orlando in mid afternoon - and couldn't understand what was happening to the aircraft.
Just concentrated on keeping the wings level on instruments - that's all I had the confidence to do even though the ASI was fluctuating between indicating a stall, and then going toward Vne.
I too am now extra cautious about IMC flying.
Hope not to experience that again, but believe we should be prepared for the unexpected. And thanks to the excellent training that is available in the UK - that should be possible for all PPL's.
rgds

[Capt Homesick]

I would guess at a blocked ASI initially- remember it needn't only be caused by ice- insects are another possible cause. The indication staying rock-steady suggests constant pressure within the pitot tube- and I would normally expect some fluctuation of aispeed indication in normal flight. Then, when the turbulence hit, the stall may have allowed stagnation pressure to drop enough that the obstruction simply fell out.
The above is simply a possibility- I wouldn't disagree with any of the other suggestions.
As to teaching failure of the pressure instruments- I used to teach it at IMC level. I seem to recall failure of Gyro instruments was referred to as "limited" panel, and failure of the pressure instruments as "partial" panel.
Thanks for the account- food for thought for all of us I think.