If an arrival time falls between two INTER periods, which one is considered valid?
Please see attached for more details

Thanks in advance
Ian

Gday Ian,

Are you wanting to know which is valid for expected weather conditions, or alternate planning.. Or both?

Either way, you'd need an alternate or holding fuel.

Hi Ian,

Just to be sure we cover everything, I'm going to break down this TAF and look at the various conditions being described in order to make sure you are clear on what is going on with it.

First off, let's consider any possible Operational Requirements (an alternate, holding fuel) for the flight arriving at 0500. When you do this, it can be helpful to simplify the conditions down to being either "Bad" or "Good".

"Bad" means you need an alternate or holding fuel, "Good" means no operational requirement applies. When changing from one type to the other and you are deciding whether you have an Operational Requirement (OPR), you must apply a 30 minute safety buffer to the forecast time of change.

So, if the weather is changing from "Bad" to "Good", you must assume the weather will change for the better 30 minutes later than the forecast predicts. If it is changing from "Good" to "Bad", you have to assume the weather will worsen 30 minutes sooner than the forecast predicts.

In your example however, the base weather for the TAF is OK but very windy, perhaps too windy for your aircraft type even though it is almost blowing straight down the runway at Merimbula.

However it is changed with the "FM 04" to be definitely "Good" weather with no OPR. The wind drops significantly and even though the cloud base is forecast BKN at 2500 these conditions would definitely not require an alternate or holding. Since the conditions could be considered to be better than before, we have to assume it will happen later than forecast. In other words, as far as our OPR is concerned, we assume the FM takes effect at 0430.

We're arriving at 0500 so we seem to be in the clear. We are well and truly in the period of "Good" weather.

So far so good. Now we move on to the INTERs you mentioned. First off, as Ellzy said, both the INTERs have an OPR. You are going to need to carry an alternate or 30 minutes holding fuel no matter when you arrive. Since we have an OPR no matter what time we arrive, there's no need to consider a 30 minute buffer for the change: we are simply going from "Bad" to "Bad". We need 30 minutes holding fuel or an alternate.

Remember, you only apply the 30 minute time buffers when you are considering whether you have an OPR for the flight.

So, we know we have an OPR but what about the actual forecast weather conditions?

Arrival is defined as arriving overhead the aerodrome, so when we say we are arriving at 0500, we mean we are arriving overhead at 0500. It makes sense to consider that the conditions will be as described in the INTER applicable as of 0500. Another way to think of it is the -TSRA conditions are forecast to stop at 0500 and the low vis, low ceiling conditions are forecast to start at 0500. If you were arriving at 0500, it only makes sense to consider the conditions that are due to start.

So, the conditions you would expect for an arrival at 0500 are:

Winds of 250/15KT and a visibility of 10km or more in light rain showers with 5-7 oktas of cloud at 2500 ft but there will be periods of up to 30 minutes duration where the visibility drops to 5km with 5-7 oktas at 1000 feet.

Isn't Met great

Cheers,

Rich

Excellent replys.

This was probably where I've been going wrong:
Remember, you only apply the 30 minute time buffers when you are considering whether you have an OPR for the flight.
I've been applying the 30min buffer for OPR's and arrival conditions.....

Many thanks Richard and Captainellzy

Thought that might have been the case, very very easy to do.

Thanks Rich for yet another detailed answer

Ellzy

Hi Rich,

I've been reading some of the old questions posted in the forum and came acorss this one where you stated,

'Remember, you only apply the 30 minute time buffers when you are considering whether you have an OPR for the flight.'

What exactly do you mean by this statement? Can you please clarify once more.

For forecast variations in a TAF (INTER and TEMPO), I've been applying the 30 minutes buffer to both sides as stated on page 7.25 of the book. Tell me where I have gone wrong.


Thank you.


Frederik

INTER and TEMPO conditions will generally only be included in a TAF if they impose an OPR, so you almost always need to add the 30 minute buffer to them. I say almost, because it may be that the INTER or TEMPO is due to wind strength, and obviously this may or may not impose an OPR depending on the runway(s) available at the destination and the aircraft type you are flying (different aircraft can handle different maximum crosswind conditions before it becomes an OPR).

Does that help?

Hi sydpilot,

Think of it this way: the 30 minutes buffers are there to protect you from being caught out by the weather and thereby running low on fuel. So, if you are flight planning and trying to decide whether you need an alternate or holding fuel based on the forecasts, you must be pessimistic and assume improvements in the weather will happen 30 minutes later than forecast and deteriorations in the weather will happen 30 minutes sooner than forecast.

However, if you are simply wanting to know (or the exam is asking) "what is the forecast weather for an arrival at time DDHHMM?", you will simply read the forecast as it stands. There is no need to apply 30 minutes buffers to the times mentioned in the forecast.

Cheers,

Rich

Thank you both Ray and Rich. I understand it better now.

It's also important to understand the difference between VARIATIONS and CHANGES.

VARIATIONS occur when the background weather remains unchanged, but the forecaster expects periods when there will be deteriorations, with a subsequent return to the background weather conditions, such as afternoon showers. These variations will be indicated by either INTER or TEMPO. When VARIATIONS are forecast, the 30 minuter buffers are applied to both the beginning and the cessation of the period. Any aircraft arriving with that period must consider the OPR for flight planing.

CHANGES occur when one set of conditions will be replaced by a new set of conditions and the weather after the change will remain as described e.g. the passage of a cold front. CHANGES are indicated as FM or BECMG. In the case of a change, the 30 minute buffer is applied only to the beginning of the change and, as Richard said, the buffer is applied before the change if the change is from 'good' to 'bad' and 30 minutes after the change if the change is from 'bad' to 'good'.