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Factors that affect the time of last light and DR
Unusualattitude
Topic Author
Unusualattitude created the topic: Factors that affect the time of last light and DR
Hi,
I came across this question regarding the time of last light.
The question is:
The time of last light is affected by
a. Latitude, date, cloud cover, reduced visibility, and terrain
b. Longitude, date, cloud cover, reduced visibility, and terrain
c. Latitude and date only
d. Latitude, longitude, date, cloud cover, reduced visibility, and terrain
From what I understand, I know that only the Latitude and date affect the duration of daylight but in this case the answer should be A because longitude does not affect the daylight at all. Am I correct?
Another question is the definition of dead reckoning: The data of dead reckoning air navigation are position, distance, direction, and time? What about speed and distance?
Cheers
Richard replied the topic: Re: Factors that affect the time of last light and DR
G'day,
The actual time of last light is only affected by date and lattitude (answer C). However, if you're talking about time of useful daylight, then A would be correct since cloud, vis and terrain (e.g. mountain shadow) will all affect the light levels and make it darker than expected - even if you still haven't reached official last light.
You are right, DR relies on position, distance, direction and time. Speed is just distance divided by time so if you know the distance and the time, you can work out the speed.
Unusualattitude replied the topic: Re: Factors that affect the time of last light and DR
But wouldn't terrain and reduced visibility have some effect on the last light. For example, if the aerodrome is to the east of the mountain, shouldn't the last light be earlier because the mountain blocks out the light from setting sun? But I think in this case, it has to do with when the sun is below six degree below the horizon.
Richard replied the topic: Re: Factors that affect the time of last light and DR
Yes, they sure would have an effect on the amount od daylight at the field but they won't change when the centre of the sun's disc is 6 degrees below the horizon. As pilot in command you will need to take such environmental factors into account when deciding the latest you can arrive at an airfield. You could plan to arrive 10 minutes before last light (and be legal) only to find that the low cloud and mountain shadow have made the aerodrome essentially dark.
It's one of those cases where being legal is not necessarily enough to be safe.
