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.
