Jan 13, 2012 - Most pilots know about aircraft flight and airframe limitations, often referred to as the "flight envelope." If you look at a graph of aircraft flight limitations the lines may appear in a rectangular shape, resembling an envelope. Also similar to an envelope are the limitations, which, if exceeded, may place the aircraft outside of an “enclosure” of safe flight, possibly jeopardizing the integrity of the airframe.

Performance Limitations

For example, aircraft have speed restrictions - fast and slow - as well as gravitational force (G's) limitations, and combinations. At one corner of the flight envelope is the maneuvering speed (V_a), which in non-technical terms provides a speed that should be used for rough air. Exceeding it in rough air or with abrupt control movements may subject the airframe to structural damage, while flying slower than V_a during rough air or with abrupt control movements may lead to a stall.

Other corners address maximum speed for structural limits of the airframe and maximum angle of attack before a stall. Service ceiling, top speed, and G limits for speeds and altitudes are also addressed. A sample theoretical flight envelope is shown below with G limitations on the vertical axis and speed depicted on the horizontal axis.

Pilots familiar with these types of limits for the aircraft they fly, and who fly within the prescribed limits, assure themselves a margin of safety. That margin was built into the design of the airplane by the manufacturer then published in the flight manual (Pilot Operating Handbook) for the benefit of pilots. If you are not familiar with the flight envelope of your aircraft review the manual to learn, or refresh your memory, on the airframe and flight limitations.

Another Flight Envelope

That’s one flight envelope, but there is another that I would encourage you to examine. That envelope is more complex and more personal for each pilot, for each flight, for each different type airplane, for each flight mission. It’s one you construct. Let’s call it the Mission Envelope.

The Mission Envelope is the combination of standards or limits that each pilot needs to assemble to remain safe. Pilots stay safe because they have identified - in advance - specific flight, weather, aircraft and personal limitations on a flight by flight basis. Hence, the name Mission Envelope.

Looking at specific examples will help clarify the details of this second envelope for safe flight. If it’s pouring rain with a low freezing level and you have a VFR aircraft not suitable for flying in icing or IFR weather and you are not rated to fly IFR, the decision not to fly that day is easy. No prudent pilot would attempt it - too risky. Let’s progress from that easy decision to other decisions.

Assume a day with good visibility, high ceilings and a 15-knot direct cross wind, forecast to increase with gusts and a 1500 foot broken ceiling later in the day. The pilot, hypothetically, plans to fly a warbird with an 18-knot crosswind limitation. OK so far? Maybe. Let’s continue.

The pilot has not flown for 60 days, will be taking a passenger, not a pilot, who is unfamiliar with the aircraft. He has lots of questions and wants to get some good photos too. More data: the pilot has 15 hours in the airplane, acquired over two years; his total flight time is 1200 hours over 40 years. He has a Private license and added an instrument ticket six years ago.

What do think? Is the pilot inside his Mission Envelope? The more data added seems to indicate the flight might represent one that would challenge, if not exceed, the pilot’s proficiency, and at least his comfort level.

Is such a flight FAA-legal? Looking at minimum standards it may be. Is it prudent? Probably not. Certainly, the risk factors would vary from pilot to pilot, but you see the challenge.

Mission Factors

The Mission Envelope is not a hard and fast set of weather minimums, looking only at ceiling, visibility, or even winds. It involves much more. For example, criteria such as the amount of sleep the pilot had, pressures he/she may face from work, spouse or finances can significantly contribute to the safety of the flight because they relate to the pilot’s ability to focus on the mission and complete it safely. Add night, a headache, hasty preparation, last minute rushing to fly, short window to complete the flight, and like issues and you’ll have a potentially hazardous flight.

The Navy Test Pilot School at Pax River has actually constructed a model to help quantify these sometimes intangible threats to a safe flight. In 1996, the Naval Safety Center, after years of mishap data collection and analysis, concluded that four out of five causal factors were related to human factors. The concept of Operational Risk Management was instituted by the military and a similar concept adopted by airlines. Conclusion: human factors are very relevant to safety.

(Notice I refer to flying warbirds as a mission, which I believe it is.) These aircraft were, in most cases, flown by military pilots—probably in their twenties or thirties, maintained by government budgets, with abundant government maintenance personnel, thoroughly trained in the aircraft’s systems, flown on specific missions to accomplish specific tasks. Whether training or battle, each mission was briefed thoroughly and flown according to the briefing, then terminated. Military personnel were usually on duty to confirm wheels were down and locked and the aircraft was configured to land. On carrier aircraft, trained LSOs even determined safe glide paths for landing, or the pilot was instructed to go-around, or “wave off” for Navy jocks.

Now, along comes some fortunate civilian pilots, with a wide range of training and experience, often serving as their own maintenance officer, with a wide range of ages—all maybe twice the age of the original military pilots—who have the freedom (it’s a great country) to fly these wonderful, high performance aircraft. Should we ignore the professional preparation of the original operators of these planes? If they found it valuable to treat the flight as a mission, why don’t we?

Mission doesn’t mean the flight has to have some historical significance or computed value. Flying over the desert to look at the Spring flowers can be a mission. The message here is preparation and making certain the aircraft and pilot are mission-capable. For some reason I am not able to cite, flights appear to be safer if there is a defined purpose, direction, even altitude and plan. You are the one who defines those goals and limits.

Defining Your Mission Envelope

In applying the concept of a Mission Envelope to your flights there are some guidelines that can make the process integrated into your flying habits.

1. Define the mission
2. Confirm aircraft capability
3. Confirm pilot capability
4. Set external standards
5. Define abort standards and procedures

The mission: Defining the mission is relatively easy, but the process contributes to safety so go along and make the determination: why are we flying? What will we accomplish? Fun is OK; photos are OK; aerobatics, if trained, is OK. Just pick something and define it and some limits.

Aircraft capability: Confirming the aircraft capability is something we have probably done many times but let’s follow through with it each time. For instance, takeoff data in hot weather or high elevations, landing in winds or with forecast ceilings, fuel duration (big issue with many of the jets), and overall mechanical condition—hydraulic reservoir full, oil clean and full, nitrogen or air pressure, if applicable. Check the accident reports; you’ll see aircraft that have been damaged when simple cautions have been overlooked, such as landing wheels up due to taking off without sufficient hydraulic fluid and no fluid or pressure in the emergency accumulator. These aircraft are durable and relatively reliable, that is, if the systems are checked and maintained. Flying frequently helps too. (I noticed on my L-29 there is a storage checklist in the maintenance manuals if the plane does not fly for 30 days.)

Pilot capability: Pilot capability is much more subjective. Some pilots are very capable after a day of work, slight headache, etc., while others are not. Substantial flight experience can deliver dividends when unforecast weather conditions appear or system emergencies surprise a pilot. This criterion of pilot readiness is worth examining thoroughly. For example, I and the formation pilots I fly with have automatic no-go standards that include weather, winds, certain maneuvers—no air show formation loops off the deck is one—that we agree not to perform. We limit our night flying to good VFR and don’t try to “punch through a hole” to get VFR in the daytime. We avoid flying formation with pilots we haven’t flown with (they may be better than we are, but we don’t take tag-alongs or impromptu formation flights). Come up with your own standards, and then look back as they prove to be valuable. I can count many instances where I was happy to be on the ground instead of risking a flight in conditions that didn’t meet my personal limits.

External standards: Flights in international environments, densely populated airspace, high-traffic corridors, areas where inexperience pilots may gravitate on the first sunny day, ignoring radio and standard arrival procedures, all constitute what I label as external factors. They exist but can be easily avoided. Before flying where the external factors represent a significant challenge, evaluate the risk and preparations for minimizing it carefully.

Define abort standards and procedures: From my observations made while training pilots at all levels, from pre-solo to current F-18 pilots, one of the maneuvers that I don’t think is applied frequently enough is the simple go-around. If you always land, regardless of alignment, speed, position down the runway, you may be letting standards slide. Going around is OK. Some of the most experienced pilots I have flown with go around more frequently than lesser experienced pilots. Have a plan to keep within good, safe limits. The same is true on a cross-country flight too. If things are looking risky—airplane consuming excessive fuel, unidentified noises, deteriorating weather, more headwind or less tailwind than planned—take command and follow a backup plan. If nothing else, land and rethink the flight. I have never regretted landing, even when the issue turned out to be nothing.

I remember the time I saw a fuel tank indicate almost empty in only a few minutes of flight, with no visible leaks or other detectable problems. I elected to land at the nearest suitable field. On landing, I discovered a slight leak in a fuel cap with a gasket that had a tiny, almost invisible crack, which permitted fuel to be siphoned out—many, many gallons of fuel—in just minutes. Didn’t regret landing, though it was tempting to blame a faulty fuel gauge and continue on.

If something looks bad, it rarely gets better on its own. Land and check it out. Divert if the place of intended landing is part of the problem.

Applying the System

Although applying the process is simple, recalling a recent situation may help illustrate the concept.
In flying a formation of single-engine jets from Alaska to California, our team had to make several decisions and then stick to our standards. Some of the issues included decisions to fly airplanes that had not flown recently, possibly through Canada, while watching VFR closely. Here’s a synopsis of the analysis final decisions agreed to by the pilots:

1. preparing to go through customs, including advanced document approval, was superior to flying where weather personnel mentioned was a constant IFR-on-short-notice area, with little backup airfields, though it was in US airspace;
2. no night flights outside US airspace and no night IFR;
3. all flights were delayed for three weeks while mechanics re-accomplished all annual inspections all the aircraft, which were then test flown and fine adjustments made so all of them worked perfectly, otherwise they were rejected;
4. all pilots had substantial jet aircraft flight experience and international experience;
5. all aircraft were equipped with duplicate GPS systems;
6. pilots were formation qualified;
7. all flights were thoroughly briefed and backup plans identified;
8. no hard schedule was set for arrival, so no risks would be taken to meet a “get-home-itis” deadline;
9. all pilots agreed that if any aircraft consumed excess fuel that plane received priority and would be identified for arrival first.

The standards served the team well. For example, on one leg the winds were 50 knots less tailwind than forecast, I diverted the flight, as was briefed as our backup plan, and we stayed overnight at a city that was clear and landed with plenty of fuel. The entire mission was successful, safe and completed without incident.

Two Envelopes; Safe Flight

By staying inside the limits of both the aircraft flight envelope and the Mission Envelope, safety will be an easy objective. Plus flying will be more fun.

Douglas Gilliss, CFII, ATP