November 2016 Issue

How Low Can You Go?

The FAA’s rules on flying airplanes at low altitude are in FAR 91.119, Minimum safe altitudes. The rule has three basic parts. First, the FAA does not want you to fly lower than “an altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.” Makes sense.

Over congested areas, which the FAA defines as any congested area of a city, town or settlement, or over any open air assembly of persons, the FAR says you must be at 1000 feet above the highest obstacle within 2000 feet of the airplane.

It should be noted the FAA’s definition of a congested area may not be the same as yours or mine. The rule of thumb is that “small, sparsely settled residential areas are settlements” for purposes of determining whether an area is congested.

Elsewhere, the agency wants you at least “500 feet above the surface, except over open water or sparsely populated areas,” and no closer than 500 feet to any person, vessel, vehicle or structure.

Low-Level Flying

It continues to be a leading cause of fatal accidents. If you’re going to do this, at least do it right.

According to the AOPA Air Safety Institute’s 25th Nall Report, which takes a detailed look at general aviation accidents that occurred in 2013, pilots are our own worst enemy. The pilot-related accident cause category far exceeds the other two broad categories—mechanical and unknown—when it comes to assigning blame for non-commercial fixed-wing events. During 2013, 74 percent of all such GA accidents were pilot-related, comprising 72.9 of all fatals for the same period. Of the pilot-related causes, maneuvering leads the pack and, as the charts on the opposite page summarize, low-level flying is a major component, associated both with loss-of-control and obstacle encounters.

The Report states, “Whether they involve in-flight losses of control or collisions with obstructions, the vast majority of fixed-wing maneuvering accidents share a common element: the sequence is initiated at low altitude. While some occur in the traffic pattern, many crashes following unintended stalls and nearly all collisions with power lines, broadcast towers, and ridgelines arise directly from the pilot’s decision to fly needlessly low in inappropriate locations, making spins unrecoverable and leaving the airplane vulnerable to obstacles that could easily have been overflown.” How can we fix this? For instance, we can’t stop pilots from engaging in low-level flying, especially since it can be a valuable mission. The problem isn’t that were flying low at low altitude. The problem is we suck at it.

Losing Control

Let’s start by defining “too low.” Many pilots may think low-level flying is a FAR violation. Not necessarily—see the sidebar on page 14. And some wags may say the only to know for sure that you’re too low is when you run into something. We’ll get to obstacles in a moment, but another way to define “too low” is in terms of whether stall recovery is likely before hitting terrain. But for those who need a number, let’s arbitrarily agree that anything under 500 feet agl is low flying.

Besides obstacles, the other way pilots get into trouble at low altitude is loss of control in-flight (LOC-I), a type of accident garnering attention in recent years. One mitigation against low-level LOC-I is a modern angle-of-attack (AoA) indicator, a device more reliably informing a pilot of when the airplane is nearing a stall than indicated airspeed alone. As envisioned by the FAA and industry, the advisory AoA is designed to help prevent low-level stalls/spins by giving the pilots additional guidance on how close their wings are to the critical (stalling) angle of attack in all attitudes, including turning flight, and is especially useful at low altitude, where there’s less room to recover. With or without an AoA indicator, it’s best to avoid the kind of maneuvering from which a stall/spin is likely.

And airspeed alone isn’t enough, since an airplane will stall at any speed. Perhaps the classic example of the low-level LOC-I accident is spotting game from the air and maintaining a steeply banked (45-60 deg.) turn for a closer look. We’ll have to increase power and/or pitch to maintain level flight, which tends to tighten the turn. As the turn tightens, more pitch/power is necessary to maintain altitude, until we start pulling some G and enter an accelerated stall. All of which could be avoided if we imposed three basic limits on our low-level maneuvering.

Piper Aztec low approach

D. Miller/Creative Commons

Maneuvering Limits

The first limit we should consider in our game-spotting scenario is bank angle. Despite what the commercial pilot flight maneuvers might require, there’s little reason to bank the airplane beyond 30 degrees intentionally. Beyond 30 degrees of bank, the airplane may require significant pitch and power inputs, beguiling us closer to that accelerated stall. We’d suggest that 30 degrees be our maneuvering bank angle limit.

low level flying accidents

aircraft accidents from low level flying

The charts track the occurrence of maneuvering accidents (top) and its sub-categories (bottom), showing that aerobatics-related maneuvering accidents are the most lethal, followed by stall/loss of control and colliding with obstacles. Flying into mountains/canyons brings up the rear.

Another limit we should consider is that no turn is allowed to progress beyond 180 degrees of heading change. Right away we eliminate the tightening-turn scenario with such a rule, but that alone won’t be enough. Once we level the wings after 180 degrees of turn, we should assess the airplane’s situation: Did we lose altitude? Did the airspeed drift from where it was when we started the turn? If the answers are yes, we need to continue straight and level until we’re satisfied speed and altitude are back where they should be. Then we can turn around (no more than 30 degrees of bank!) and fly back to the point of interest and do it again. (After all, you have an airplane—you can go anywhere you want, right?)

The third limit is airspeed. It should be Mama Bear-right on entering the maneuver and monitored throughout. We shouldn’t execute another of our 180-degree/30-degress-of-bank turns until we’ve recovered altitude and any loss from this target airspeed. In no case should this speed be greater than the published maneuvering speed (VA), of course.

Many pilots may suggest 1.3 VS1, the bottom of the green arc, as a good speed to fly. We think that puts us too close to the flaps-up stalling speed; best to use something a bit faster, so we’ll settle on 1.5 VS1 as our slowest available speed. So if you have an airplane with a 130-knot VA and a 60-knot VS1, the acceptable range of speeds at which to fly this low-level turn would be no slower than 90 KIAS (1.5 x 60). Somewhere between 90 and 130 KIAS is the speed we’d use to fly our low-level turn. Any time our speed drops below 90 KIAS, we level the wings and discontinue the turning maneuver in favor of straight-and-level flight until we regain the target airspeed and position ourself to make another 180-degree turn over our point of interest.

No—these maneuvering limits aren’t etched in stone, nor are they guaranteed to keep us out of trouble. But any airplane we’re likely to fly should easily handle a 180-degree heading change at no more than a 30-degree bank and 1.5 VS1. And since the objective here is to reliably avoid a low-level stall/spin accident, it also places us at a comfortable margin above the bottom of the airspeed indicator’s green arc. By avoiding the trap of low-level, steeply banked circling, we likely can also avoid the stall/spin we risk.

Among pilot-related accidents in 2013, three basic types comprised a significant portion of the fatalities, according to AOPA ASI: weather, maneuvering and other.

Weather was the most lethal, involving 30 fatals out of 41 (73 percent) total accidents, followed closely by maneuvering (27 fatals out of 48 total accidents, or 56 percent). Other pilot-related accidents in 2013 involved 27 fatals out of 117 total, or 23 percent. See charts at right.

low airplane approach

D. Miller/Creative Commons

low airplane approach

D. Miller/Creative Commons

low airplane approach

D. Miller/Creative Commons

Obstacle Avoidance

At this point, you may have noticed I haven’t specified a minimum altitude below which we shouldn’t be flying. Earlier, we defined anything under 500 feet agl as being low-level. But what about 400 feet? 250? 50?

We’d argue that any altitude complying with FAR 91.119, as discussed above, is adequate. We’d also argue you’d be nuts to descend below 500 feet agl without a really good, mission-related reason, so we’ve managed to cover all the bases. Finally, we’d suggest the primary question you need to address isn’t how low you can go but what kind of obstacle-avoidance survey or assessment of the target area are you going to conduct before getting there.

The 25th Nall Report includes a case study on maneuvering, highlighting an October 4, 2013, accident involving a Cessna 340A that collided with a radio tower during a low, high-speed pass near a recreational facility. According to the Report, the accident airplane already made one low pass over the facility. On its second one, it “flew straight and level into a 50-foot-high radio tower, severing the top 10 feet of the structure and folding up the airplane’s right wing.” All four aboard the twin Cessna were killed when it crashed into trees, inverted, some 700 feet beyond the tower’s base.

It’s unreasonable to expect all low-level obstacles you might encounter to be charted. Nevertheless, studying the available VFR charts must be part of any planning you perform prior to flying low. But that likely won’t be enough if you plan to fly at less than 200 feet agl. Instead, you’ll need to do something in the way of what we’d call a site survey: Fly over the area at a higher altitude first, looking for and noting the locations of any obstacles posing a risk to your proposed low-level flight. Even then, there’s no guarantee you’ll identify everything you can fly into. With that in mind, we’ll simply argue that enough is enough, and that you have no business flying that low. The likelihood of encountering an unseen or unknown obstruction is just too great, and the potential benefits from your planned low-level flying—if they exist at all—cannot outweigh the risks.

Charting Obstacles

If you absolutely, positively have to fly low, you need to research what obstacles, if any, exist along your low-level flight path. That’s especially tough if you plan to cruise at low level; such a mission is better described as scud running and is a whole ’nuther topic.

Either way, to learn about potential obstacles, you’ll likely coax your EFB app into displaying a visual chart of the target area and study it. The only problem with this plan is obstruction charting is not 100-percent foolproof from the pilot’s standpoint.

According to the FAA’s Aeronautical Chart User’s Guide, sectional and terminal charts typically show man-made obstacles extending up more than 200 feet agl “unless they appear in yellow city tint.” In other words, a) obstacles topping out at less than 200 feet agl aren’t charted and, b) obstacles in urban areas may not be charted at all. World aeronautical charts (WACs), which are being discontinued, only chart those obstacles of 500 feet agl and higher.

The Guide also helpfully adds, “Features considered to be hazardous obstacles to low-level flight are: smokestacks, tanks, factories, lookout towers, and antennas, etc.” At right are some examples of the symbols used in the FAA’s visual charts to denote obstacles.

Stay High

The accident record fairly well describes the two main risks of low-level flying: losing control and flying into obstacles. We’ve explored ways to mitigate these issues by adopting flight procedures and maneuvering limits you can use to ensure the airplane remains under control throughout, and we’ve tried to point out there’s simply no reliable, foolproof way to identify and avoid all potential obstacles.

We’ve also presumed your desire to fly low over a certain location has a mission-oriented purpose and is known before you take off, allowing for at least some planning, a presumption often not borne out by the facts. If that’s the case, and the mission can be accomplished through some other means, like using a drone for photography, that’s the solution we’d recommend.

Engaging in intentional low-level flying often is a spur-of-the-moment thing accident pilots simply haven’t thought out, and for which they aren’t prepared. Don’t be that pilot.

Jeb Burnside is this magazine’s editor-in-chief. He’s a 3200-hour airline transport pilot and aircraft owner.