Why is scud-running necessarily dangerous?

Question

(Disclaimer: Please do not take the below as an endorsement of scud-running.)

Scud-running is when a pilot flying in poor weather operates their aircraft at a lower-than-normal height above terrain in order to remain clear of clouds and maintain visual contact with the ground. This is generally considered a dangerous practice, having been linked to a number of fatal crashes.

What I’m curious about is why scud-running is necessarily so dangerous; by definition, it involves keeping one’s aircraft in clear air and maintaining visual reference with the ground, which should (in theory, at least) enable a pilot to maintain terrain clearance and avoid becoming disoriented (the latter of which is especially important for a non-instrument-rated pilot, for whom entry into cloud, unless followed by immediate exit from same, is not generally survivable). Although flying closer to the ground does bring the aircraft closer to ground obstructions, such as trees, tall buildings, and cell-phone towers, these usually rise no more than 100 feet or so from ground level (a height low enough that one would be mere seconds from impacting the ground in any case), while the locations and heights of the rare exceptions are clearly marked on VFR charts, which should make it easy to give them a wide berth.

Even if the height between the ground and the clouds were to narrow to the point that an aircraft, to remain clear of cloud, would have to fly low enough to risk hitting low-height obstacles, this condition would be expected to become evident in plenty of time to turn away (a sudden step-down in the cloud base over a fairly-short distance is meteorologically unlikely, and would be visually obvious from a distance; a gradual lowering of the cloud base would still be obvious [from objects on the ground getting visibly closer and larger] long before one would risk striking low obstacles; and a terrain feature raising the ground into the clouds, such as a mountain, would be visible from a long way off).

Finally, one particular variant of scud-running is widely-used even in commercial passenger aviation: in a contact approach, a pilot gets ATC permission to discontinue a standard instrument approach and instead proceed at low altitude to the destination airport by visual reference to the ground, as long as the aircraft can stay out of the clouds and maintain at-least-1-mile visibility (equal to the bare VFR minima for uncontrolled airspace), and this is apparently safe enough for the FAA not to have banned contact approaches.

So why is scud-running necessarily a dangerous practice?

Answer 1

I once worked as a bush pilot on floats (my Most Fun Year), flying back and forth from a base in a large mining town to various lodges and fishing camps, ferrying wealthy Americans to their summer "camps" (cottages), and so on. I spent many hours scud running (which I would define as flying under ceilings of less than 1000 ft and visibility of less than 3 mi).

The bush operator I was with was hard core; we used 50000:1 scale topographical maps to get around, where you might have to take 3 maps to cover a trip (aeronautical charts don't have the details you needed) and the flying in bad weather was straight pilotage, navigating from lake to lake, finger tracing the route on the map (It was 1990; GPS didn't exist, and Loran C was a thing and the Cessna 185 I was flying had it, but I didn't trust it. The only radio nav aid I ever used was the ADF, which I would tune to an AM radio station in town to listen to music and provide a bearing if necessary).

At the start of the season I would turn around and return on a trip when the visibility dropped below a couple miles. By mid season I was flying in 1 mile and 500 ft, often cruising at 300 ft off the trees and a couple hundred feet below the cloud base to this place and that.

It is relatively safe in certain circumstances:

• You know the operating area well, especially the hazards like towers and cable spans, etc. My scud running trips were usually to places I'd been to many times and I got comfortable going in pretty bad conditions if I knew the route well. I would use higher limits if the trip was to somewhere new.
• You have the ability to land anywhere if necessary. Being on floats, with a land-able lake within 5-10 miles at any one time, provided ready escapes if things closed down.
• It's much safer in the morning than the afternoon, because the dew point spread was increasing, not decreasing. You avoided scud running in late afternoon whenever possible because whatever clag you were in was guaranteed to get worse as the temperature dropped. When checking weather, the dew point spread was the most important bit of data.
• When flying along river valleys, stay on the right side - other airplanes may be coming the other way, and you try to stay above the valley tops to avoid cable spans. I would turn around if the cloud ceiling dropped to less than a couple hundred feet above the hill tops on each side of a valley and forced me down into the valley itself.

Now that is while bush flying on floats, over the Canadian Shield with a zillion lakes every which way.

Scud running in "civilization"? Suicidal. Towers everywhere. 200 ft towers, 500 ft towers. Towers are shown on charts, but new ones are popping up constantly. If it's hilly country, you just stay away from the tallest hill tops, but in the flat lands they can be anywhere.

Power lines everywhere. Clouds merging with hills. Just too many things to run into. Landing options few and far between (without damaging the plane at best) if the weather closes in. I felt safe doing it on floats with lakes everywhere in a relatively remote area; I would never do it on wheels or in settled areas, for all the tea in China.

Yes contact approaches are allowed, but the exposure period is short, and they are still considered the most dangerous way to complete an IFR flight if the conditions are below normal VFR.