More Information‎ > ‎Newsletters‎ > ‎

Feature Article 2

FEATURE ARTICLE

Max Trescott, 2008 National CFI of the Year mtrescott@comcast.net

Unstabilized Approaches Cost Two Pilots Big Bucks

You may have heard of a stabilized approach and know that you should be established on one prior to landing. But you might not know all of the elements of a stabilized approach, or the potential consequences of an unstable approach. Last year, two West Valley pilots had to dig deep into their wallets to pay for damage resulting from landing after unstable approaches. Both considered going around, but didn’t. Here are their stories and details on how to fly a stabilized approach.

The first incident occurred with a pilot I had recently checked out in a Diamond DA40.  Soon after, he flew the plane to Southern California on a flight lasting several hours. When he finally arrived late in the afternoon, he was tired and very ready to be done flying. On final, he was high and fast and realized he would be landing on the second half of the runway. However, he felt he could still make the landing work. After landing long, he applied the brakes very hard, and he was able to bring the aircraft to a safe stop before the end of the runway.

That might have been the end of the story, except I was the next person to fly the plane with a client after the plane returned to Palo Alto. As is our custom before climbing into the plane, we rolled the plane forward while inspecting the main tires for flat spots. Because of a bright sun and shadow, I couldn’t see if there were any flat spots, so I got on the ground in front of a tire to watch it as the client pushed the plane. I’m glad I took the extra time to check, as I discovered the biggest flat spot I had ever seen in my long flying career! The other tire had an equally large flat spot. The pilot paid $800 for new tires and labor to change them.

The second pilot didn’t get off so easily. After a month of not flying, he took a Cessna 182 to Half Moon Bay to practice landings. After four or five good landings at Half Moon Bay, he returned to Palo Alto. Approaching the field, he relaxed as he was returning to his “home field,” with which he was very familiar. Ironically, relaxing is the wrong thing to do at Palo Alto, as the runway is among the shortest ones that most pilots encounter.

Like our first pilot, this pilot ended up high and fast, and never really considered going around because he felt he could save the landing. He landed long and hard, resulting in a large bounce. When he touched down the second time, his nose was low, the nose gear collapsed, and the prop was destroyed as the airplane skidded to a stop in a nose-low attitude. The damage, which included replacing a near TBO engine, propeller, wing tip and the bottom of the engine compartment, is estimated to require over a $100,000 in repairs. Like the first incident, this one could have been avoided by a timely go around.

So what is a stabilized approach, and why does it matter? Cirrus Aircraft’s Flight Operations Manual gives a good description. It says: “A stabilized approach is characterized by a constant angle and constant rate of descent approach profile ending near the touch-down point. Stabilized approach criteria apply to all approaches including practice power-off approaches.“


It goes on to say that for VFR landings, an “approach is considered stabilized when all of the following criteria are achieved by 500' AGL:

• Proper airspeed,

• Correct flight path,

• Correct aircraft configuration for phase of flight,

• Appropriate power setting for aircraft configuration,

• Normal angle and rate of descent,

• Only minor corrections are required to correct deviations. A go-around must be executed if the above conditions are not met and the aircraft is not stabilized by 500' AGL.”

 

In both incidents cited, the aircraft was not on a proper flight path, as it wasn’t aimed at the first third of the runway.  And since they were high on final, neither pilot was able to use a normal descent angle or a normal rate of descent.

But why were both aircraft too high? This often occurs during a normal pattern when an aircraft is too high as it turns onto base. It also is likely to occur on a long, straight-in approach, as pilots are often late to slow to approach speed on straight-in approaches.

The FAA Airplane Flying Handbook says: “The placement of the base leg is one of the more important judgments made by the pilot in any landing approach. The pilot must accurately judge the altitude and distance from which a gradual, stabilized descent results in landing at the desired spot. The distance depends on the altitude of the base leg, the effect of wind, and the amount of wing flaps used. When there is a strong wind on final approach or the flaps are used to produce a steep angle of descent, the base leg must be positioned closer to the approach end of the runway than would be required with a light wind or no flaps.” 

To prevent being high on final, I check my altitude throughout the pattern, to see if it’s close to what I think it should be. For example, before turning base, I check to see that I’ve descended about 100 feet below traffic pattern altitude in an 800-foot pattern, or about 200 feet below traffic pattern altitude for a 1000-foot pattern. In addition, if I’m not already descending at about 500 fpm, I correct my descent rate as I prepare to turn base. If I haven’t lost this much altitude before the base turn, or have a low descent rate, I often end up high on final, increasing the chances of an unstable approach.

Many pilots relax when doing a long, straight-in approach, since it should be easier than flying the traffic pattern, but they often end up high and fast. To avoid this, I mentally unwrap the traffic pattern and imagine I’m still flying it as I’m flying a straight-in approach. To keep it simple, let’s estimate that the downwind (after you pass the numbers), base, and final legs are each one-mile long. Then when flying a straight-in approach, imagine when you’re three miles out that you’ve just passed the numbers on a downwind leg, and make sure your aircraft is configured for a downwind leg, and is at the proper altitude and speed for that leg. At two miles out, configure the aircraft and fly it at the speed you would use if you were on base. Finally at one mile out, fly the aircraft in the configuration and speed you would use if you had just rolled out onto final.

Another helpful number to remember is 320 feet per nautical mile. That’s how much altitude you need to lose every mile when descending on a 3-degree glide slope. So when you’re three miles out on a straight-in approach, you should be at about 1,000 feet.

A handy reference when landing on runway 31 at Palo Alto is that the Amphitheater is about 2.5 nautical miles from the runway. So for a 3-degree descent path, you would cross the amphitheater at 800 feet. But actually the VASI at Palo Alto is set for a nonstandard 4-degree angle. So to descend on the VASI into Palo Alto on a straight-in approach, cross the Amphitheater at 1100 feet.

Use these tips, and avoid being high and fast on final. But if you do end up on an unstabilized approach, just GO-AROUND! Failing to do so can have a serious impact upon your wallet.


Stabilized Approach Definition

A stabilized approach is critical to a safe, successful landing. A stabilized approach is characterized by a constant angle and constant rate of descent approach profile ending near the touch-down point. Stabilized approach criteria apply to all approaches including practice power-off approaches.


VFR Stabilized Approach Definition

All briefings and appropriate checklists should be completed by 500' AGL in visual conditions. A VFR approach is considered stabilized when all of the following criteria are achieved by 500' AGL:

• Proper airspeed,

• Correct flight path,

• Correct aircraft configuration for phase of flight,

• Appropriate power setting for aircraft configuration,

• Normal angle and rate of descent,

• Only minor corrections are required to correct deviations. A go-around must be executed if the above conditions are not met and the aircraft is not stabilized by 500' AGL.


FAA Airplane Flying Handbook

The placement of the base leg is one of the more important judgments made by the pilot in any landing approach. [Figure 8-1] The pilot must accurately judge the altitude and distance from which a gradual, stabilized descent results in landing at the desired spot. The distance depends on the altitude of the base leg, the effect of wind, and the amount of wing flaps used. When there is a strong wind on final approach or the flaps are used to produce a steep angle of descent, the base leg must be positioned closer to the approach end of the runway than would be required with a light wind or no flaps

A stabilized descent angle is controlled throughout the approach so that the airplane lands in the center of the first third of the runway. The descent angle is affected by all four fundamental forces that act on an airplane (lift, drag, thrust, and weight). If all the forces are constant, the descent angle is constant in a no-wind condition. The pilot controls these forces by adjusting the airspeed, attitude, power, and drag (flaps or forward slip). The wind also plays a prominent part in the gliding distance over the ground [Figure 8-2]; the pilot does not have control over the wind but corrects for its effect on the airplane’s descent by appropriate pitch and power adjustments. Considering the factors that affect the descent angle on the final approach, for all practical purposes at a given pitch attitude there is only one power setting for one airspeed, one flap setting, and one wind condition. A change in any one of these variables requires an appropriate coordinated change in the other controllable variables. For example, if the pitch attitude is raised too high without an increase of power, the airplane settles very rapidly and touches down short of the desired spot. For this reason, never try to stretch a glide by applying back-elevator pressure alone to reach the desired landing spot. This shortens the gliding distance if power is not added simultaneously. The proper angle of descent and airspeed is maintained by coordinating pitch attitude changes and power changes. The objective of a good, stabilized final approach is to descend at an angle and airspeed that permits the airplane to reach the desired touchdown point at an airspeed that results in minimum floating just before touchdown; in essence, a semi-stalled condition. To accomplish this, it is essential that both the descent angle and the airspeed be accurately controlled. Since on a normal approach the power setting is not fixed as in a power-off approach, the power and pitch attitude are adjusted simultaneously as necessary to control the airspeed and the descent angle, or to attain the desired altitudes along the approach path. By lowering the nose and reducing power to keep approach airspeed constant, a descent at a higher rate can be made to correct for being too high in the approach. This is one reason for performing approaches with partial power; if the approach is too high, merely lower the nose and reduce the power. When the approach is too low, add power and raise the nose.

Comments