CAVOK or CAVNOTOK?

[radarcontrol]

Hi all.

Im in the process of attempting to start my PPL (A). For the last week I've been chasing that elusive first lesson with no success having had 3 cancellations to date. All of which have been perfectly understandable.. first low cloud and drizzle, then high wind..

However, my lesson this afternoon was cancelled due to "poor visibility". This is confusing me somewhat. The wx at the airport this morning was poor, 2500 metres in mist. It then steadily increased to 4000m, then 8KM, then 15KM to a point just before my scheduled start time at which the ATIS gave, "CAVOK" which i understand to mean, ceiling and visibility OK.. cloud higher than 5000' and no significant weather to aviation. Yet, my instructor called to let me know that the wx was "crap" and that we wouldn't be flying.

I have absolutely no reason to disbelieve him - the horizon still looked rarther blurred and I am but a beginner - but I did wonder what the point of the term CAVOK was.. when clearly the visibility was not "OK" ?

Any help in explaning much appreciated!!

RC

[FNG]

For the early lessons you need a good horizon in order to explore the effects of controls and to get used to the various pictures out of the front. Yesterday the haze was grotty, and today it is almost certainly worse. If you cannot get high enough to beat the clag (and controlled airspace and/or clapped out training aircraft may may this difficult and/or slow), then you may not gain much at the early stage from flying in haze. Later on you should do lots of it as it is common on seemingly lovely summer days. If the horizon looks blurry on the ground, it will look like sh** in the air (in fact, it will look like nothing: it just won't be there).

Either that, or your instructor had too many of your northern beers yesterday.

Enjoy your training! Are you at Leeds or Sherburn? (I assume Leeds as you mention an ATIS).

[radarcontrol]

I am indeed at Leeds... and learning fast about the poor wx I have heard so much about!

RC

[FullyFlapped]

Radar, I learned to fly at Leeds, and also keep my plane there.

Which school are you learning at, and which day are we talking of ?

FF:ok:

[FlyingForFun]

RC,

CAVOK means the visibility is (as you say) 10km or more.

But remember that the term CAVOK is used for terminal weather reports and forecasts - intended to be used when making a decision about whether to take off or to land. 10km vis is more than adequate for any pilot to be able to take off or land safely, so there is no need to report visibilities any higher than that.

When you're talking about flying away from the airfield, though, other factors come into consideration. And for the very early stages of learning to fly, having a good horizon is essential. This might mean that you need visibility of 20-30km.

The best place to get this information is from a pilot who has just landed. Failing that, MetForm 215 is available on the Met Office's website, and this will give the en-route visibility. But even that won't show you how high the haze goes.

You will become more and more familiar with this as you gain experience. In the mean time, is it possible for you to ride along in the back seat during someone else's lesson? This way, even if you get nothing else from it, you will be able to see how good or bad the horizon is, and relate this to what the forecast says.

FFF
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[Charlie Zulu]

Hi Radarcontrol,

The CAVOK statement sometimes misleads...

Firstly, CAVOK requires the following conditions.

Visibility is 10Km or greater, there is no cloud below 5000' or below the MSA, whichever is greater AND there is no CB in the vicinity and no significant weather phenomena is present at the airfield.

However, CAVOK can remain in place within the METAR/ATIS until one of the following occurs.

The visibility falls to 5KM or less, the cloud ceiling to 1500' or below and/or significant weather occurs.

So even if it is saying CAVOK, the reality is the weather could have been 6KM with a 1600' Cloud Base and it could still legitimately be called CAVOK.

Just because they say its CAVOK doesn't mean its nice weather to go flying, never mind basic attitude lessons for your PPL where you need a clearly defined horizon...

Hope this helps.

Best wishes,

Charlie Zulu.

[radarcontrol]

Fully flapped...

It was this afternoon (08/06/04). I'm learning at multiflight - or trying to :rolleyes: . Bloomin weather.

CharlieZulu....

That seems ridiculous to me. What's the point of having an ATIS report CAVOK when visibility could be 5000 metres?! Surely that doesn't fit in with the notion that it must be 10KM or more to be reported CAVOK. Seems to me it would be far more useful for everyone if the actual visibility was always included.

I have 3 lessons left this week... tommorow, Thurday and Friday. Here's hoping.

RC

[bookworm]

I think CZ is talking about something slightly different.

A METAR will report CAVOK when the vis is greater than 10 km, cloud is above 5000 ft or the highest.. ya dee ya dee ya. If the vis drops or cloud appears, the next METAR observation will not be CAVOK.

However, if CAVOK appears on the TAF, the Met Office will not rush out an amendment just because some cloud is likely to appear at 4500 ft, in fact not unless the conditions fall considerably below CAVOK.

The problem with METARs is that they were designed with commercial air transport in mind, and most of the time they're flying IFR. 10 km and 40 km are both CAVOK, but the consequences for visual navigation are very different. 10 km means that you can see at most about 3 minutes flying time ahead, and certainly doesn't guarantee you a good horizon. While I understand your frustration at the cancellation, you may have been saved an expensive and unhelpful hour struggling with the conditions.

[Charlie Zulu]

Hi Bookworm,

Sorry about the misleading information, but that is what I was led to believe whilst training for my PPL all those years ago. Oops, I'm doing Met at ATPL level in Module 1 so my understanding should be rectified by the end of October! ;)

Best wishes,

Charlie Zulu.

[radarcontrol]

Bookworm...

Many thanks. That has clarified things for me significantly. I wouldn't debate that the weather today has been pretty horrible in terms of getting airborne. Just got slightly confused with my terminology!

RC

[Keef]

Came home last Sunday from France, with CAVOK reported all the way. When we got to Southend, it was still "CAVOK", but the missus in the right seat commented about how murky and hazy it was.

I didn't mind, because I know the area well, but it would not have been a useful experience for someone taking the early stages of lessons.

[DRJAD]

Flew from Sherburn yesterday late afternoon, having been staring at the weather changes all day, and wearing out the various websites to get TAFs and METARs for the routes involved.

The haze was certainly at that time very obscuring of the horizon, though vertical viz. was acceptable. The eternal problem of looking towards a milky murk towards the sun; though horizontal visibility 'downsun' was not too good either. Not much observable difference at FL50 compared with 1000' in the circuit. Wouldn't have been comfortable for air exercises needing the horizon picture.

Before I got my licence, I had some training at LBA, and gained the impression of fairly frequent wx cancellations: after all the elevation there is something like 682', so lurking that much further away from mean sea level will make some difference!

[FullyFlapped]

Yup, DRJAD has it bang on. Learning at Leeds means putting up with lots of cancellations due WX when, on the drive up there, everything seems fine ...

Apart from the general viz/murk problems, the elevation and geography also mean you're going to be very used to X-wind landings by the time you're done !

Good luck - PM me if you fancy a spin sometime. Perhaps we could organise a trip on a day when the WX looks good on the ground, but is actually not all that clever viz wise - then you can see the difference for yourself. The controllers might sound grumpy, but I know they love it really when us little guys ask for an ILS !

FF:ok:

[Flyin'Dutch']

Whilst I understand that there can be situations were the weather is flyable for some PPL training and not for others I find it difficult to understand that one would elect not to get a PPL stude in on a day which is reported to be CAVOK.

There is plenty of other stuff to be taught (preflighting and some basics on theory) and doing half an hour even with a less than perfect horizon is better from an encouragement point of view than cancelling yet another first lesson.

Sorry, not a great decision on his part in my books.

FD

[boomerangben]

Frustrating isn't it. Spent alot of time flying in that part of the world and you do get plenty of gold fish bowl days. But when the weather is good, you can't beat flying over God's own County! (Well apart from the West Coast of Scotland, where God has his second home!)

Your instructor probably saved you an unpleasant flight - we used to cancel loads of low time students on hazy days, mostly because of the horizon, but also hazy days are good days for chukkers!!! Not nice on your first lesson.

Managed to get above the crud the other day (not bad for a helicopter pilot whose nose starts to bleed at 4000'!) Gorgeous.