Hypoxic Hypoxia

Hypoxic Hypoxia

Hypoxic hypoxia is the result of low oxygen levels available to the entire body. A blocked airway and drowning are  examples of how the lungs can be deprived of oxygen.

The reduction in partial pressure of oxygen at high altitude is an appropriate example for pilots.

Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases. 

As the airplane climbs during flight, the percentage of each gas in the atmosphere remains the same, but there are fewer molecules available. Why?

Daltons Law of Partial Pressure states that each gas exerts a unique pressure at STP. For oxygen, at STP, this is 20% of 760 mm Hg for an Atmosphere, or a partial pressure of 159 mm Hg. 

At 18,000 feet the atmospheric pressure is one half what it is at sea level. 360 mm Hg comes to mind. Take 20% of 360 and the drop in oxygen partial pressure goes to 72 mm Hg.

Your body needs approximately 100 mm Hg partial pressure at the alveolar sacs to replenish to adequate levels of oxygen in the blood. Why the drop from 159 mm Hg to 100 mm Hg at sea level.

Your respiratory system is like a tube that extends from your nose to the surface where a "gas exchange" can take place. In the non-exchange areas (like the trachea) the oxygen from the atmosphere mixes with the carbon dioxide and water vapor present in the "dead" air. This reduces the partial pressure of oxygen to 100 mm Hg. (Ain't this fun!!)

When the air mixes with carbon dioxide and water vapor, in the physical respiratory system track, before it gets to the alveoli, the partial pressure of oxygen is too low to keep you awake for flying. You will pass out in approximately thirty minutes without supplemental oxygen.

The decrease in the partial pressure of oxygen prevents an adequate number of oxygen molecules reaching the alveoli.

This is hypoxic hypoxia.

Hypoxia - Symptoms

Hypoxia Symptoms

There are several types of hypoxia but each causes the brain to receive less oxygen than normal. This is hypoxia and, as you receive less oxygen, each of us that experience hypoxia don't realize the symptoms that warn us we are impaired.

A general feeling that "all is well" and we think nothing is wrong because we feel so good. If the lack of available oxygen decreases more your arms and legs don't seem as responsive and coordinated flying starts to disappear.

Most folks may or may not share the following symptoms that reflect you are hypoxic.

• Cyanosis - (blue fingernails and lips)
• Headache
• You don't react well
• Your judgement is impaired
• Feeling of "all is well"
• Vision begins to deteriorate
• You become sleepy
• You become lightheaded
• You experience dizziness
• You have tingling sensations in your hands and feet
• Numbness

If the hypoxia worsens the field of vision narrows. (it is like looking through a tube - peripheral vision disappears). Your ability to interpret what the instruments show dissolves. You have a false sense of security and are deceived into thinking everything is normal.

The treatment for hypoxia, while flying, includes flying at lower altitudes or using supplemental oxygen.

All pilots are susceptible to hypoxia regardless of physical endurance or acclimatization. If you plan to fly at high altitudes you must use supplemental oxygen to avoid the symptoms of hypoxia.

The term "time of useful consciousness" describes the maximum time the pilot has to make rational life-saving decisions and  carry them out at a given altitude without supplemental oxygen. As your altitude increases above 10,000 feet the symptoms of hypoxia increase rapidly. The time of useful consciousness rapidly decreases.

Since symptoms of hypoxia can be different for each individual, the ability to recognize hypoxia can be greatly improved by experiencing and witnessing the effects of it during an altitude chamber “flight.” (hyporbaric chamber)

www.faa.gov/pilots/training/airman_education/aerospace_physiology/index.cfm.

The Federal Aviation Administration (FAA) provides this opportunity through aviation physiology training, which is conducted at the FAA CAMI and at many military facilities across the United States. For information about the FAA’s one-day physiological training course with altitude chamber and vertigo demonstrations, visit the FAA web site above. 

Landing Errors due to Optical Illusions

Landing Illusions

Illusions created in flying an airplane can kill. This post provides pilots with information to recognize conditions that lead to optical illusions and how to cope while flying.

• You can anticipate the possibility of visual illusions when you fly into a new airport destination.  Especially at night and/or when adverse weather conditions exist. Make sure you consult Jepp Charts on runway slope, terrain and lighting.
• Increase your scan of the altimeter during all approaches in day or night conditions.
• If possible, consult with a pilot that actually landed at the new airport destination. He may provide important information. If not, try and inspect the the airport yourself before flying there.
• Use Visual Approach Slope Indicators (VASI) or Precision Approach Path Indicators (PAPI).These systems help visual reference in adverse conditions where the horizon is obscured or visually missing. Another source is an electronic glideslope, if available.
• Use a visual descent point (VDP) found on many non-precision instrument approach procedure charts. These help protect you from beginning your approach too soon or too late.
• If emergency or other activity distracts you from your usual approach procedures the chance of an accident  increases.
• Maintain, by practice, proficiency in landing procedures.

In addition to the sensory illusions due to misleading inputs to the inner ear vestibular system, a pilot may also encounter various visual illusions during flight.

Illusions rank among the most common factors cited as contributing to fatal aviation accidents. That is one of the safety factors stressed in my posts.

Sloping cloud formations, an obscured horizon, a dark scene spread with ground lights and stars and certain geometric patterns of ground light can create illusions of not being aligned correctly with the actual horizon. these may cause you to approach too high or too low. Serious matters depending on obstructions before the runway or running out of runway if too high.

Various surface features and atmospheric conditions encountered in landing can create illusions of being on the wrong approach path. Landing errors due to these illusions can be prevented by anticipating them during approaches, inspecting unfamiliar airports before landing, using electronic glideslope or VASI systems when available, and maintaining proficiency in landing procedures.

Like a Sinking Plane - Does it Create Lift

Sinking Planes - Do they create Lift?

Lets exaggerate a possible situation how an airplane in flight adjusts its own lift to its own weight.

First have the airplane flying straight and level at cruising speed. The airplanes lift and weight are in equilibrium. More lift than weight it would balloon upward. More weight than lift it would sink. Since it is flying straight and level it is "in balance."

Keeping the planes attitude exactly as it was in straight and level flight reduce the speed by twenty miles per hour. Using the controls keep the nose of the aircraft, relative to the horizon, exactly where it was in straight and level flight.

To see what happens, in just a fraction of time, let these events seem like they take minutes to occur.

If everything remains the same (attitude, relative wind, angle of attack) the wings move through the air at a slower speed and develop less lift. 

If you have a 3,500 pound SeaBee and it is only developing 3,000 pounds of lift force it does what a stone does - it drops.

It doesn't drop nose down because you, the pilot, are maintaining the attitude exactly as it was in straight and level flight. You are not allowing the attitude to change with respect to the horizon. The plane does not descend, it falls.

A plane, like a falling stone, falls slowly at first, then falls faster and faster because of gravity. If nothing else happens gravity acts on the 500 pound difference between the weight of the SeaBee and the 3000 pounds of lift force until the airplane crashes into the ground.

Something does happen to stop its falling like a rock. Remember, the SeaBee is sometimes referred to as "The Flying Rock" for it tendency to enter a steep glide angle after you cut power for a landing.

All of the above happens so fast (remember we delayed the events) the plane falls only a few inches. It finds a cushion of new lift and what creates this cushion is the aircraft's sinking. How does this happen?

As the airplane moves downward through the air as well as forward the relative wind now blows at the wings as well from in front. The Angle of Attack becomes greater even when the attitude of the plane hasn't changed. Angle of Attack is the angle at which the wing meets the air. The greater the Angle of Attack means that more lift is created. As the plane reaches a sinking speed at which the Angle of Attack is so high that lift forces are now equal to its weight and the plane acts like an aircraft again.

If the power is maintained at the level you reduced it to the aircraft recovered its equilibrium in descent and it will stay in descent. It is a steady descent.

This is a "Power Descent" much like a modern commercial jet that cuts power many miles from the airport as it begins it approach.

You see, a sinking plane does create lift!

A New Look at Dalton's Law of the Partial Pressures of Gases

Dalton's Law of Partial Pressures - New Look

Why do we seem "winded" on a hot, humid Summer day? Why can't we cool down? Good questions. At Standard Temperature and Pressure, at Sea Level, one Atmosphere consists of nitrogen, oxygen, carbon dioxide, water vapor and trace gases. If you took the partial pressures of each gas they would exert their pressures independently of each other but, in total, would add up to 760 mm Hg at STP.

Each of these gases make up a  specific percentage of the whole atmosphere. This doesn't change. Nitrogen 79% , oxygen 20% and all the rest 1%. If water vapor partial pressure increases the partial pressures of the other gases is reduced to reflect the presence of water vapor in the atmosphere. The slight reduction in oxygen's partial pressure is enough to "tire" us a bit.

In dry climates the humidity is low. In sunny South Carolina the humidity is quite high. The "Comfort Index" indicates conditions are very uncomfortable. Why is that?

Upon evaporation of water (changing from a liquid state to a gaseous state) the evaporation could use up 580 +- calories per gram of water evaporated. In total, that is a "cooling effect" that makes you feel good and normal body temperature is a good thing to control. Think heat prostration and heat stroke.

If the water vapor content is high, evaporation ceases or decreases. You can't convert water to water vapor to create the cooling effect.

Notice, all of these changes are taking place at sea level, with slight adjustments for the elevation.