VFR Visibility Minina

[shumway76]

Note: The altitude mentioned is just a value for example sake as different countries may have different altitudes for VFR visibility minima

Within uncontrolled airspace:
VFR visibility minima above 10,000ft is 8km
VFR visibility minima below 10,000ft is 5km

Why the is the visibility minima at lower altitude lower?
I would assume at lower altitude you need better visibility since you'd expect more traffic at lower altitudes, but it looks like it's the other way around.

So my question is, why is the visibility minima at lower altitude lesser than at higher altitudes?

[Fl1ingfrog]

At the lower altitudes the earth is the main reference for attitude and distance assessments even though this is always difficult when airborne. The higher you go the less the earth remains a good reference and so the greater the margins you need to allow from other aircraft and cloud. This is a gradual factor of course but for law and flight rules you need to provide a fixed datum: i.e. 5000 ft and 10,000 ft as an example. As you say, many countries have taken a different view as to where to best set the altitudes/levels. So, it's not that scientific but has to do with practicality.

You have to allow that at the time all this was decided in the 1950s, commercial operators would have exerted much pressure on deciding the VFR levels. A large number of transport aircraft did not have IMC capabilities. Anti/de-icing came later and compared to today these aircraft had poor instrumentation. There was very little radar available so aircraft sought to remain in vmc clear of cloud often at the lower vmc altitudes for a mix of reasons.

[VFR-Seek and Destroy]

Originally Posted by shumway76

Note: The altitude mentioned is just a value for example sake as different countries may have different altitudes for VFR visibility minima

Within uncontrolled airspace:
VFR visibility minima above 10,000ft is 8km
VFR visibility minima below 10,000ft is 5km

Why the is the visibility minima at lower altitude lower?
I would assume at lower altitude you need better visibility since you'd expect more traffic at lower altitudes, but it looks like it's the other way around.

So my question is, why is the visibility minima at lower altitude lesser than at higher altitudes?

Generally below 10,000 feet you are restricted to 250kts.
Therefore above 10,000 feet traffic can be moving faster so one would need better visibility to "see and avoid" other traffic.

[squidie]

Originally Posted by shumway76

Note: The altitude mentioned is just a value for example sake as different countries may have different altitudes for VFR visibility minima

Within uncontrolled airspace:
VFR visibility minima above 10,000ft is 8km
VFR visibility minima below 10,000ft is 5km

Why the is the visibility minima at lower altitude lower?
I would assume at lower altitude you need better visibility since you'd expect more traffic at lower altitudes, but it looks like it's the other way around.

So my question is, why is the visibility minima at lower altitude lesser than at higher altitudes?

VFR heavily relies on visual flying. So below 10,000ft you would rely a lot more on closer features such as towns, terrain and roads for example. The higher you are the more you relay on feature further away, such as cities and landscapes and mountains for example.

[oggers]

Generally below 10,000 feet you are restricted to 250kts.

Is the best answer. The 250kt/10000 rule came later than visibility minima but the principle that you fly faster at altitude is a physical fact ie for any given indicated airspeed your TAS increases with altitude.

The other two answers can be seen to be red-herrings when you consider the simple fact that you can be vmc without having the ground in sight at all. If the intention of the rule was to facilitate visual navigation at high altitude there would be no vmc on-top. Also, the ground 8 km ahead is likely to be under the nose in most piston singles from 10 000 plus feet up.

The reason for this rule is to give more time to see and avoid.