Night instruction

[FlyingForFun]

A quote from my copy of Trevor Thom volume 2, "Aviation Law (and Meteorology)". This is from the 5th revised edition, published 1999, at the top of page 10:Night is defined for the Rules of the Air as being: from 30 minutes after sunset; until 30 minutes before sunrise.
This differs from the definition of night reqiured for night training flights, which is when the sun exceeds twelve degrees below the horizon (reference Air Navigation Order, Article 118 and Schedule 8).I am trying to find out more about this definition of night which is required for night training, but without much success. ANO Article 118 describes the powers of an authorised person to prevent an aircraft from flying... not very relevant. And Schedule 8, although it talks in some detail about licenses and ratings, does not give any specific definition of night to be used for night training.

In fact, in searching through the whole of the ANO, the only reference I can find to the sun being 12 degrees below the horizon is for night currency for helicoptors and gyroplanes. I also haven't been able to find a reference to this rule in any other air law books.

My best guess is that this is a pre-JAR rule which no longer exists, but I don't have an up-to-date copy of the Trevor Thom book to hand to confirm whether this rule has now been removed. In talking to other instructors, I have found that many weren't aware of the rule, but those who were aware of it don't know any more than me. Does anyone know for certain what the status of this rule is?

Thanks,

FFF
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[homeguard]

FFF

I think that with further digging that you will find that the 30min rule is simply to define when the flight Rules; VFR/IFR can be/must be applied and the rule is taken with the surface as a datum.

The 12 degrees below the horizon (the horizon being the furthest point that can be seen) is to define the conditions that must exist for 'Night Training' to take place. As you surely know, at altitude it may still be quite bright although on the surface it is quite dark. This often catches many inexperienced pilots out. Having said that it is, on calculation, amounting to being the same for most of us. Sunset is defined as is twilight and therefore night. The rate at which we rotate and our movement around the sun would mean that thirty minutes is about equal to 12 degrees of change following sunset.

I'm sure that this will be explained better and that somewhere there is in the thomes the details that you seek. I wait with interest.

[FlyingForFun]

Homeguard, that's interesting.

I had always assumed (not sure where I got this assumption from, I guess someone must have told it to me at some point) that "the sun 12 degrees below the horizon" was some time later than official night (twilight) - around 30 minutes later.

This seemed to make sense to me. After twilight, it is still possible to see the surface of the runway, and use that (rather than the runway lights) as a visual reference for landing. Thus, it made sense to wait until some time after twilight before teaching people to fly at night, such that the only visual reference they have available is the runway lights.

But what you are saying is that this is not the case at all - that this 12-degree thing is near enough the same as twilight, but deals with the difference in appearance of the sun at altitude? In which case, my understanding of the rules has been wrong for quite some time! Oh well, it won't be the first time.....

FFF
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[homeguard]

One explanation but there may be many.

'Sunset is generally considered to take place when the top of the Suns disk passes below the horison.

Since the atmosphere scatters sunlight, the sky does not become dark instantly at sunset; there is a period of twilight.

During civil twilight, it is still light enough to carry on ordinary activities out-of-doors; this continues until the Sun's altitude is minus 6°.

During nautical twilight, it is dark enough to see the brighter stars, but still light enough to see the horizon, enabling sailors to measure stellar altitudes for navigation; this continues until the Sun's altitude is minus 12°.

During astronomical twilight, the sky is still too light for making reliable astronomical observations; this continues until the Sun's altitude is minus 18°. Once the Sun is more than 18° below the horizon, we have astronomical darkness. The same pattern of twilights repeats, in reverse, before sunrise.'

The calculations within the almanac used for aviation appear to be calculated using only Lat and Long and take no consideration of altitude. Maybe somebody thought of this and therefore, for training purposes, considered that the actual visual conditions should be taken into account, in order that a realistic visual experience of night was always encounted.

Hopefully an expert with the full knowledge of the history will chip in and illuminate us.

[Keygrip]

astronomical darkness
Best name for a rock band I've ever heard.

[G-SPOTs Lost]

Keygrip - yeh groovy man!!!!! ;)

FFF why not just ring 8212 and ask?? :D :D :D

[DFC]

As far as I am aware, the two different starts of night in the UK disapeared with the introduction of JAR-FCL.

Since then, the definition of night for licensing and training puroses is the local definition of night i.e. 30 after sunset to 30 before sunrise.

The ANO was completely revised not that long ago. Thus all books prior to that should be treated with caution.

Regards,

DFC