Minimums, Maximums, & Margins

Things to live by.

Minimums

Minimums

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There's plenty of information on the operating limitations of an airplane. A VG diagram (in some applications called a VN diagram) shows many things including stalling speed, maneuvering speed, maximum allowable speed, maximum indicated airspeed in rough air and maximum allowable G loading, both positive and negative. Operate within the parameters of the VG diagram and you are flying the airplane within its envelope. The closer you fly to the edges of the envelope, the lower your margins.

Pilots don't have to learn to read VG diagrams because this information is shown in other ways. Airspeed indicator markings and operating limitations provide the information that we need to operate within the envelope.

That is the simple part. Everything fades to a shade of gray when we mix up the other minimums and maximums we have to deal with while flying, especially in regard to weather, and consider them in relation to the margins that we want to always maintain.

Let's look at those weather minimums first. For the VFR pilot, minimum weather is pretty easy to judge. Just look out the windshield for word on whether or not you can see enough to fly visually. The picture is there on a continuous basis. A pilot who continues VFR into adverse weather conditions almost always does so with full knowledge that he is trespassing on Mother Nature. The only way a pilot can be "trapped" by weather is to knowingly fly into foul conditions.

Living with minimums should be as simple for the IFR pilot. There are minimum altitudes prescribed for all published routes. Controllers have minimum altitudes for you when not on published routes. Many pilots fly with terrain information on the panel should anything go awry.

The real sticky question arises when flying that instrument approach in scuzzy weather. There are minimum altitudes to follow there, but for virtually all approaches there's that dicey part where we have to switch from electronic guidance to visual flying. If an autopilot is used, as it should be on every approach in low weather, we also have to switch to hand flying for that last little bit. A lot of things change in a little time and the margins are slim at best as we deal with this.

One thing that is often bandied about is the concept of "personal minimums." In theory that means you don't plan flights into areas unless the weather is reported and forecast to be better than the published minimums. It's a way of increasing margins. Trouble is, the weather doesn't always cooperate and reported or forecast weather that is above your personal minimums might in fact be below the published minimums for that approach when you get there. I recall one approach where the forecast and reported weather was well above minimums but when I was flying at the minimum descent altitude I was still on top of clouds. That happens more often than you might think.

In an accident where the pilot of a Columbia (now Cessna) 400 crashed on a foggy night, the NTSB included some technical data about the automatic weather observation system (AWOS) at the airport where the airplane crashed. The report said that the ceiling would have been updated every 20 minutes, the visibility every eight to 10 minutes, the temperature and dew point every four to five minutes, and the altimeter setting and winds every 30 seconds.

The report went on to say that the equipment was near the glideslope antenna with the visibility referenced to the north (the runway was 33) and taken between two arms that are six feet apart. The ceiling height is measured vertically from the unit and is massaged a bit for changes over time.

With a report of a mile and a quarter visibility and a 500-foot ceiling the suggestion to this pilot was of conditions above minimums for the ILS, which had a three-fourths of a mile visibility requirement and a 200-foot decision height.

That snapshot of the weather from the AWOS apparently didn't tell the whole, or even part, of the story. Witnesses reported that there was fog around. The pilot flew the ILS but, according to the NTSB, he failed to execute the missed approach and crashed off to one side of the runway about 75 percent of the way down the runway.

The moral to that story is to never believe that there is any close correlation between reported weather and the weather at the point where you reach the decision altitude on a precision approach (ILS or LPV) or the minimum descent altitude on a non-precision approach. That point in space is not where the reported weather comes from. Where there can be conditions of uniform visibility and ceiling over an area, that is not likely if the terrain is undulating and the temperature and dew point are close together. If the weather observation is made by a live person it might be more revealing. In some cases, an observer can supplement an automatic report.

Another lesson that can be learned from this is that scud or patchy fog can effectively make the visibility and ceiling at or near zero. When we are coming up on a minimum altitude, our possible view of the runway is on a slant and one little bit of scud at 100 feet could keep you from seeing the runway from the point or altitude where a decision must be made. There is probably nothing that leads a pilot into the weeds more quickly than a "now you see it, now you don't" condition.

Under Part 91, not for hire, the visibility is our official minimum and the rule says that flight visibility is all that counts. So if we are compelled to consider that the weather can be worse on the final approach course than is reported at the airport, maybe it can also be better. We are free to fly an approach regardless of reported weather to see if indeed the flight visibility is such that we can see the runway at the appropriate time. We are also free to crash, as many pilots have done, trying to make it into an airport with a reported quarter of a mile visibility and a vertical visibility of 100 feet. To land, all you have to do to be legal is swear that the flight visibility was equal to the published minimums and that you saw the runway (or other applicable things) at the proper time.

The only way to stay out of trouble on an approach is to follow the letter of the law and not leave a published minimum altitude on an approach until and unless the runway or other specified items are in view. If they are not, then a missed approach has to start immediately and anything that happens to come into view after the missed approach begins can't be used as a reason to change the plan and try to salvage the approach. Once a missed approach is begun, it needs to be continued.

If a pilot wants to establish a personal minimum for weather on approaches, it might be best to not leave that minimum altitude unless the runway itself is in clear view.

While this was being prepared there were two IFR accidents on ILS approaches, both at night. One airplane hit two miles short of the runway, the other hit "approximately 3,575 feet" short, according to the NTSB.

Accidents like this are far from uncommon on precision approaches. Why do pilots go below the electronic glideslope and crash? In both of these cases it happened before the pilot even reached the point where a decision had to be made. We can only imagine what would make a pilot go below the electronic glideslope well before reaching the point on the approach slope where the decision altitude would have been reached. Perhaps the indication was misread. More likely, something was seen that prompted the pilot to go lower for a better look. That is never a good idea.

The margins on an instrument approach are not great and they become ever less the closer we get to the appropriate minimum altitude. Pilots need to fly with that in mind.

Back to VFR. In the recent past there have been a number of serious accidents in pistons and turbine airplanes that have come during botched approaches and landings. When the event turns ugly, it is usually a product of the approach being too high and too fast, the pilot forcing the airplane onto the runway, the pilot deciding there is not enough runway left to stop, and the airplane crashing off the end of the runway because the go-around was started too late for any chance of success. All the margins were given away.

For a remedy to this we have but to look at an ILS approach. The decision height is 200 feet above the ground. On any approach on a clear day, that IFR minimum can be used as a defining point in determining if the margins are being maintained on the approach. At that time, if the airplane is on an electronic glideslope or the pilot is using a VASI and the indication is correct, then the altitude is correct. If the airspeed is correct then the margins are as good as they get. If the airplane is high or fast and the runway length is the least critical, nobody can know how much, if any, of the margins will remain if the approach is continued. That means that it should be abandoned.

When we look at takeoff and landing distances in the Pilot's Operating Handbook, the values reflected there are minimums. They might be accurate if you do everything perfectly, and if the engine is strong as new for takeoff or the brakes are as good as new for landing. Prudent pilots add margins. To keep it simple, I like to double the POH numbers for required runway length. Over the years, that has kept me from breaking a sweat on takeoffs and landings.

Certainly if you are looking at a takeoff and you calculate that the distance over a 50-foot obstacle shown in the POH is barely adequate, then you are flying with no margins and the chance of success is not good. The same thing is true of a landing. No margins, no good.

The one thing the accident record illustrates is that if on landing you put the wheels on the ground, you should let the wheels stay on the ground. There are that relative lot of serious accidents related to go-arounds started after a touchdown with a questionable amount of runway available for stopping. There are also a lot of accidents where airplanes run off the end of the runway but the main casualty in those is usually the pilot's ego. All accidents have the potential of being serious, and there's usually a point in the sequence of events where the pilot has a choice between a decision that results in a serious crash and one that leads to a fender-bender. Making the best choice depends on whether or not the pilot has given adequate advance thought to such situations and is locked and loaded to do the best thing.

There are a lot more choices on minimums, maximums and margins. The minimum level of pilot proficiency is defined in the FARs. So many takeoffs and landings and so many instrument approaches in a specified time define the FAA's version of current, but a pilot who is satisfied with being minimally current is really signing up for second-best. Going beyond the minimum requirements on anything means the margins have been increased.

Mac wrote at length about wind in his January column. And an important question has always been about how much surface wind is too much wind. The only guidance here is the maximum demonstrated crosswind that is not a limitation. The pilot of a Cessna 172 found his maximum wind on 11/14/08. According to the FAA, the wind was from 320 degrees at 32 knots gusting to 45 when the airplane flipped while taxiing. Could a 172 be operated in such a wind without flipping? Maybe, maybe not. It's a cinch that the technique would have to be perfect for it to work and that a wind limit should be below that value. As with weather, though, wind is free to do as it pleases regardless of the forecast, and what you see is what you get.

It is an acknowledged fact that the margins are smallest when taking off, approaching and landing, and greatest when flying along in cruise flight in good weather. Cruising puts every element comfortably within the envelope.

Even at cruise, though, an untoward event can suck the air right out of those margins. Say the engine fails on a single or one engine fails on a twin. The powerless forced landing in a single and the engine out approach to a runway in a twin are quite different maneuvers, but they have a lot in common. The margins are thin because the available energy is either none or half. Either is a bad deal. The low-speed edge of the envelope is critical in both cases and the final outcome in either relates directly to how well the pilot does at maintaining that razor-thin margin.

Margins are nice things to have. One that I always maintained is in relation to airspeed. I never ever flew the airspeed into the yellow arc, which is for smooth air only. I just never wanted to depend on air staying smooth, so when the airspeed approached the yellow I let the streak down my back of the same color prevail.