Cruising Altitude

When flying anywhere you need to climb, cruise, and then descend. But you must comply with the rules and fly at certain predetermined altitudes.

The Rules: Neodd and Sweven

When you are flying above 3000 feet AGL you must fly at an even-numbered thousand feet if you are traveling west. That is what sweven means. If you are traveling directly South, or on any Westerly heading then fly on the even-numbered thousands.

Conversely, if you are traveling East or directly North, fly at an odd thousand (neodd).

For IFR flights to the East you will fly at 5000 or 7000 or 9000 feet, etc….

For VFR flights you must be 500 feet above these altitudes. So a VFR flight to the West would cruise at 4500 or 6500 or 8500 feet, etc….

These rules are for cruising altitude, meaning that if you are flying up above 3000 to practice an emergency descent then you can just climb to whatever altitude you want.

Why is this rule in place?

This rule is a bit of a compromise between safety and simplicity.

When two planes approach head-on their closure speed, the speed at which they are approaching each other, is very high. Even a relatively slow 152 will approach another 152 at around 180 knots TAS if they are head-on. For faster planes like an arrow traveling at 150 knots the closure speed is 300 knots!

It is safer to fly at these altitudes because VFR planes flying East will always be 1000 feet vertically separated from VFR planes flying West. Furthermore, they will be separated by 500 feet from all IFR traffic.

This is great, but there is a problem! What if planes approach nearly head-on but both traveling East? One of them could be traveling 010 degrees (which is East of North) and the other could be flying at 170 (which is East of South).

This is where the compromise comes in. The rule could split the compass into 4 segments but then it would be more complicated and difficult to remember.

Always stay vigilant looking for traffic that might be climbing/descending, not following the rules, or might be at a near head-on angle.

6 Types of Flaps

Flaps are very useful for giving an airplane good handling characteristics at low speed. They are vitally important for giving fast planes the ability to go slow for takeoff and landing.

There are 6 types of flaps

  1. Plain Flaps
  2. Split Flaps
  3. Slotted Flaps
  4. Fowler Flaps
  5. Slots
  6. Slats

Plain Flaps

A simple hinge at the rear of the wing is used to make plain flaps. They are easy to design but they can’t provide much lift before the drag increases very significantly.

Since increasing drag is one of the purposes of a flap the performance is not too bad for landing. When taking off though, drag is not desired.

The reason this flap has so much drag is because the air from above the wing tends to separate and become turbulent when it has to flow downwards at such a steep angle. Think of it like a car on the highway having to make a sharp turn to stay on the road. This airflow separation is like that car spinning out because it can’t make the turn.

Turbulent airflow separation above the wing reduces lift.

Split Flaps

The split flap surface is actually below the wing and pushes down out of the bottom. It does provide some lift but it creates a lot of drag and is almost like a speed brake under the wing.

Slotted Flaps

This is the most common type of flap because it is relatively simple to design and build but provides a huge benefit over the plain flap. The slot created when the flap extends allows air to flow from under the wing.

This airflow provides a cushion for the air from on top of the wing to keep it from separating. It also flows along the top of the flap surface. These two benefits combine to create a lot of extra lift.

Fowler Flaps

Fowler flaps are complex but provide a lot of benefit. They act like slotted flaps opening up a channel for air, but they also slide outwards as depicted below. By sliding out from the wing they greatly increase wing area.

Put simply, they make the wing bigger. A bigger wing means lots of lift!

Most modern airliners use double or triple slotted fowler flaps. These have several flap surfaces that extend out from each other as the flaps are lowered creating a much larger wing with several slots for air to pass through.


This modification to the leading edge of the wing provides a channel of air that is pushed up over the wing and channeled towards the upper surface. At low speeds, this simple fixed device can increase the critical angle of attack (the angle at which the wing stalls). This means that the plane can fly much slower without stalling.


Slats are another type of leading edge flap. They slide down at low speeds and provide a large increase in lift like the fixed slot. The benefit of the slat is that it is retractable and won’t create extra drag at high speeds because it slides up into the wing surface.

Flight Instruments: Airspeed Indicator

The speed of an aircraft through the air determines its performance in many ways.


How does the indicator work?

A basic airspeed indicator is a mechanical device that compares pressure from the pitot tube to pressure from the static port. The static port is mounted sideways with a hole the doesn’t face directly into the oncoming air. This way it gets a “static” measure of the air pressure. The pitot tube has a hole that does face into the airstream, so it has oncoming air forced directly into it.

The air from the pitot tube fills a diaphragm (like an accordion) and makes it expand. The air from the static port fills the gauge around the diaphragm and pushes it to contract. As the diaphragm expands and contracts it pushes a needle that we see on the instrument.

Airspeed Indicator (Source: Wikipedia)

The airspeed indicator is very reliable but there are a few things that can go wrong.

Reading the airspeed indicator

The airspeed indicator is fairly self-explanatory to read. The most common mistake is not paying attention to units. Sometimes the instrument will measure miles per hour instead of knots. Always make sure you know which one you are looking at.

The indicator can also have some error, especially at high angles of attack. The manufacturer will often publish a calibrated airspeed table to help you determine the difference.

6 Types of Airspeed

1. Indicated: IAS

Indicated airspeed is the speed that the plane “feels”. It might help to think of it as the number of air molecules hitting the plane. This is the speed that matters for the performance of the plane. It can be read directly on the airspeed indicator.

2. True: TAS

As you climb the air gets thinner. As the air gets thinner there are fewer air molecules in a given volume of air. This allows the aircraft to fly faster.

For example, if your plane has enough power to fly at 100 knots and you maintain 100 knots while climbing your true airspeed will increase. True airspeed is your actual speed through the mass of air. As you climb and the air thins out, if you are still at 100 knots then you are still encountering the same amount of air over time, but since that air is spread out over a longer distance you are flying at a faster speed. True airspeed is the same as groundspeed if there is absolutely no wind.

This bonus in speed and better fuel economy are the reasons that planes bother to climb all the way up to high altitudes.

3. Calibrated: CAS

Airspeed indicators aren’t perfect. When flaps are down or the plane is at a high angle of attack the airspeed indicator may be off by several knots. This error is studied and a placard is provided with the correct numbers. So calibrated airspeed is more precise than indicated airspeed but it is not displayed directly on the airspeed indicator.

4. Ground Speed: GS

This is not an airspeed, but it is worth including here. Ground speed is the speed that really matters for getting somewhere, it is very simply your speed over the ground. It is equal to your true airspeed plus or minus a tailwind or headwind.

When there is enough wind it is possible to gain an enormous amount of extra speed. This is why jets love to take advantage of the jet stream where the wind speed can often exceed 100 knots.

It is also possible to make a plane stop or fly backward. See the video below that illustrates this concept.

5. Equivalent Airspeed: EAS

Equivalent airspeed is a further correction of calibrated airspeed that corrects for airspeed indicator errors due to compressibility. It is most prominent at high altitudes and high speeds. Modern planes that can reach these altitudes and speeds generally have an air data computer that handles the calculation of EAS but a simple performance chart can be used as well. For light aircraft, EAS is generally ignored because it is very close to being the same as CAS.

6. Mach Number: M

Aircraft that fly at higher altitudes and speeds, like jets, generally refer to their speed in terms of mach number. This speed is measured as the ratio of the speed of sound. For example, mach 1 means you are flying at the speed of sound, and mach .5 means you are flying at half the speed of sound.

Pilots generally pronounce mach numbers like “mach point seven five”, or “mach point eight” for M.75 and M.8, respectively.

Most planes fly at subsonic speeds, less than the speed of sound.

Faster planes like the Concorde, and some military fighters and bombers can fly at transonic speeds, at the speed of sound. They can then accelerate to supersonic speeds great than the speed of sound.

Anything greater than M5.0 is considered to be a hypersonic speed. Hypersonic aircraft are certainly being studied but as far as I know, there aren’t any flying.

As an aircraft reaches higher altitudes the mach number is used to measure speed instead of IAS. An aircraft is limited in IAS by aerodynamic pressure and in mach number it is limited by aerodynamic shock waves. Since there are two different limits they both need to be considered.

For example, a Boeing 757 has a Vmo (maximum operating airspeed) of 350 knots and an Mmo (maximum mach number) of M0.86.

At a low altitude near sea level M.86 is 568 knots, well beyond the Vmo limit. However, at a high altitude like 40,000 feet M.86 is approximately 262 knots, well below the Vmo limit. This is why pilots will reference IAS at low altitudes and mach number at high altitudes. During climb there is a crossover altitude at which the transition is made from thinking in IAS to thinking in mach.