Weather Theory

Understanding the weather is a key element of knowing what we, as pilots, are up against.  It can seem abstract at first, so let's take a look at the big picture...

Atmosphere
Its purpose is to absorb energy from the sun and provide a protected/moderate climate for life on Earth.

Composition: Nitrogen = 78%, Oxygen 21%, Argon and other gases make up remaining 1%.

There are 4 distinct layers within the atmosphere going from the surface to 350 miles away.

The Troposphere extends from SL to 20,000 ft. above the poles and up to 48,000 ft. over equatorial regions.  Most weather (clouds, storms, temperature changes) occur within this layer.  Standard lapse rate of 2ºC per 1000 ft exists and pressure decreases about 1" Hg per 1000 ft.

The Tropopause is a boundary layer which traps moisture and weather in the troposphere.

The upper layers have little influence on weather and are considered to be generally stable, though the atmosphere itself is in constant motion.

•Unequal heating of the surface creates most "weather."
•Warm air rises because air molecules spread apart when heated.  Expanding air is less dense and lighter.
•Unequal heating also changes air pressure (molecules have weight).  Pressure differs based on temperature, altitude, and air density.

Coriolis Effect - Generally areas of high pressure at poles want to go to areas of low pressure around the equator.  This is interrupted by rotation of the earth.  This force deflects air to the right in the Northern Hemisphere.

•The wind forces and actions are governed by 3 Cells
-Hadley Cell  (near equator)
-Polar Cell (near poles)
-Ferrel Cell (in between)

Atmospheric Pressure
In the U.S., our measurements of barometric pressure are in inches of Mercury (Hg).  Standards were developed for a reference point.
29.92" Hg=Standard Pressure or
1013.2 millibars (mb).
Standard Temperature = 15ºC

Temperature, altitude, and density can affect pressure.

•Remember that while there is still 78% oxygen at 10,000 ft, the pressure on that partial of air is significantly less - thus leading to risk factors like hypoxia.

Wind / Currents
Because air seeks out low pressure (and travels from areas of high pressure to low pressure), and because the earth's rotation deflects these currents to the right (in the northern hemisphere), air will flow Clockwise around a High Pressure System and Counter-Clockwise around a Low Pressure System.  Understanding this can help maintain favorable winds while enroute.

Convective Currents
Different surfaces radiate different amounts of heat.  Convective currents cause bumpy, turbulent air.  Land heats faster than water, so these currents are often found near land masses near water.  Circulation patterns are not the only cause of turbulence.  Obstructions ranging from airport hangars to mountains can reroute the flow of air, in a violent manner.
•Leeward sides of mountains are especially hazardous due to the strong downward pressure and eddies.

Low-Level Wind Shear
This is a hazardous condition that can occur with passing fronts, thunderstorms, or temperature inversions.

•Microbursts last about 15 mins, but can produce up to 6,000 fpm downdrafts, and take place often in areas of less than one mile.
•LLWAS systems detect wind changes of greater than 15 kts.


Stability
Stability deals with atmosphere's ability to resist vertical motion (convectivity).  When air rises, it enters an area of lower pressure and expands to a larger volume.  The expansion on molecules causes the temperature to lower.
•Water Vapor is lighter than air, thus moisture decreases air density, causing it to rise.  Moist air cools slower and is thus more unstable (requires more vertical movement to cool)
•Dry adiabatic lapse rate is 3ºC per 1000ft
•Moist adiabatic lapse rate varies from 1.1ºC to 2.8ºC per 1000 ft

Inversions
When temperature increases with altitude gain.  Top of inversion acts as lid, pressing down and keeping lower altitudes stable.

Temperature/Dewpoint/Humidity
•Relative Humidity refers to how much moisture a parcel of air is holding compared to how much it could hold
•Dewpoint is temperature of the air, where it could hold no more moisture (clouds, fog, or precipitation result)
•Dewpoint & Temperature converge at a rate of 4.4ºF per 1000 ft

Fog
Fog is one result of saturation of air.  There are numerous types resulting from varying causes.

Radiation Fog - Often occurs in low-lying areas like mountain valleys.  Forms at night when ground cools rapidly due to terrestrial radiation and surrounding air reaches dew point.

Advection Fog - occurs when winds push coastal air over cooler ground.

Upslope Fog - occurs when stable air is forced up sloping land feature.  Requires wind to form and can last for days.

Steam Fog - forms when cold, dry air moves over warm water.  As the water evaporates, fog forms.

Ice Fog - forms in cold weather when water vapor forms directly into ice crystals.

Clouds

Clouds are another result from a temperature/dewpoint convergence.  There are varied cloud types based on height, shape, and behavior.  They are separated into Low, Middle, or High clouds.

Cumulonimbus clouds are probably the greatest cloud hazard to pilots.  Because they contain extensive vertical development, they are extremely turbulent and can produce hailstones, lightning, tornadoes, and other hazards to flight.

Thunderstorms
There are 3 distinct stages of a thunderstorm= Cumulus, Mature, Dissipating.
Cumulus consists entirely of updrafts.  These strong updrafts prevent moisture from falling.
Mature - drops of moisture become too heavy and downward motion begins.   Precipitation falls.
Dissipating - Cloud is raining itself out, characterized entirely by downdrafts.

Air Masses

Air Masses are classified according to regions where they originate.  They take on characteristics of surrounding area.
•Air masses moving over warmer surface results in convective currents and instability.
•Air masses moving over cooler surfaces creates stable conditions with poor surface visibility.

Fronts
An air mass moving across water/land will eventually come into contact with another mass with different characteristics.  The boundary layer is a front.  There are 4 types: Warm, Cold, Stationary, Occluded.

Warm Fronts - warm mass of air replaces cooler air.  They move slowly (10-25mph).  Slides over cooler air and gradually pushes it out.  Often contains high humidity.  Precipitation is probable as is a reduction in visibility.

Cold Fronts - mass of cold, dense, and stable air advances and replaces warmer air.  Move rapidly (25-30mph).  They remain close to ground and act as plow, sliding under warmer air.  Generally more volatile than a warm front passage.  Fast moving cold fronts can produce squall line thunderstorms and turbulent winds.

Stationary Fronts - occur when two air masses are relatively equal and weather is a mixture of warm/cold fronts

Occluded Front - When a fast moving cold front catches up with a warm front.  Two types:
Cold Front Occlusion - When cold front is colder than air ahead of the warm front.  Cold air replaces the cool air and forces warm front aloft

Warm Front Occlusion - When air ahead of warm front is colder than air of the cold front.  Cold front rides up and over wam front.  Generally more unstable.

Squall Line
A narrow band of active thunderstorms that often develops ahead of a cold front.  Forms rapidly and often contains steady-state thunderstorms, which are extremely hazardous to aircraft.

Tornadoes
Thunderstorms that draw air into cloud bases when there is an initial rotation.  Tornadoes occur in both isolated and squall line thunderstorms.

Hail
Supercooled drops above freezing level begin to freeze.  When enough ice has latched on to the parcel, it becomes too heavy and falls.

Airworthiness

Required Instruments for Day/Night VFR
Your airplane may not have these systems (manifold pressure and landing gear indicator etc., but eventually at the Commercial level, the following acronym is better to memorize . . .


•TOMATOFLAMES

Tachometer
Oil Temperature Gauges
Manifold Pressure Gauge
Airspeed Indicator
Temp. (for each liquid cooled engine)
Oil Pressure Gauges
Fuel Gauges
Landing Gear Position Indicator
Altimeter
Magnetic Compass
ELT
Seatbelts

After 1996, Anti-collision light system required.

•Beyond these day VFR requirements, the nature of the flight may require further equipment.  ie: Departing Long Beach requires radios due to Class D airspace and Mode C Transponder due to being within 30NM ring of LAX.

•Flotation gear for each occupant and signaling device if aircraft is operated for hire over water beyond power-off gliding distance from shore.

•Night Requirements = FLAPS


Fuses (Circuit Breakers).  If fuses, 3 of each kind required.
Landing Light
Anti-Collision Light (Beacon)
Position Lights (Nav Lights)
Source of Power (Alternator or Generator)

Inoperative Equipment


•FAR 91.213 describes acceptable methods for operation of aircraft with inop equipment.

-Operation with Min. Equipment List (MEL) = specific inoperative equipment document for particular make/model of aircraft.  Generally means plane can safely be operated within appropriate operations limitations, a transfer of function to another operating component, refernce to other instruments or components

-Operation without MEL = Plane may continue to be airworthy if inop equipment is not part of VFR-day type certification instruments and equipment, not indicated as required on the aircraft's equipment list, not required by FAR, not required by AD
INOP instruments or equipment must be removed from airplane with cockpit control placarded and maintenance properly recorded, or deactivated and placarded INOP.

Special Flight Permit
This may be done despite provisions listed in FAR 91.213.  Given if aircraft cannot meet applicable airworthiness, but is deemed safe to fly to a base where repairs can be made etc.  (FAR 21.197).

Obtaining Special Flight Permit - Contact FSDO with written request indicating
-Purpose of flight
-Proposed Itinerary
-Crew required
-Ways plane does not comply with airworthiness requirements
-Any restriction that is necessary for safe operation
-Other info considered necessary by FAA for purpose of prescribing operating limitations

Maintenance Records
Acronym: AV1ATE

Annual Inspection (conducted by A&P mechanic who also possesses an Inspection Authorization (IA) certificate
VORs - Checked every 30 days.  This is primarily for IFR requirements.
100 Hour Inspection - Required for commercial (charter, flight training, other for-hire business).  100 hour inspection essentially same thing as annual.  Can be exceeded by no more than 10 hours in order to transport aircraft to maintenance location where inspection can be performed.  Those hours over the 100 hr work against the next 100 hour inspection.  Further inspections may be required depending on specific make and model of aircraft.
Altimeter/Pitot-Static System - Must be checked every 24 calendar months
Transponder - Checked ever 24 calendar months
ELT - Inspected every 12 calendar months for 1) Installation, 2) Corrosion, 3) Operation of controls and crash sensor, 4) Sufficient signal radiated from antenna.   ELT battery must be replaced after half its useful life has expired, or after 1 hour of cumulative use.

•Owner/Operator is responsible for maintaining airplane in an airworthy condition and for ensuring compliance with all pertinent ADs.  Operator can include PIC.

•Preventitive Maintenance is minor or simple preservation operations.  Allowed items are in 14 CFR Part 43, Appendix A(c)
Entries must include 1) Description (changed oil at 2,345 hours), 2) Date of completion of work, 3) Pilot's name, signature, certificate number, and type of certificate held

Airworthiness Directives (ADs)
Correction of unsafe conditions found in an aircraft, engine, or other component.  14 CFR 39 define authority.  May be emergency ADs and require immediate compliance before next flight, or may have more leniency if they are less urgent.

•ADs are regulatory and must be complied with

Aircraft Documents
These should be checked at beginning of preflight to ensure airworthiness.  The documents are as follows...
ARROW

Airworthiness Certificate - Issued by rep of FAA after aircraft has been inspected and found to be in condition for safe operation.  Standard certificate issued for aircraft type certificated in the normal, utility, acrobatic, commuter, transport categories.  Airworthiness certificate

Registration Certificate - Before being able to fly legally, it must be registered with the FAA Aircraft Registry.  Cannot be used in foreign countries, if ownership is transferred, if certificate holder loses U.S. Citizenship, if aircraft is destroyed, or registration canceled by written request.  If any of these occurs, must notify FAA in Oklahoma City.  Recently, in an attempt to update their records, the FAA requires that all registrations be re-registered on a triennial basis.

Operating Manual (Flight Manual/POH) - This must be located in the aircraft and readily available.  Its sections are typically . . .
1) General - Basic description of airframe and powerpant, dimensions, definitions
2) Limitations - Operating limitations (Airspeed, Manifold Pressure etc.), instrument markings, color-coding, placards
3) Emergency Procedures - Checklists prescribing recommended procedure.
4) Normal Procedures - Airspeeds for normal ops, checklists, engine-starting procedures etc.
5) Performance - Performance charts, tables, stall speeds in varied configurations, takeoff/landing distance data
6) Weight and Balance/Equipment List - Info required to calculate weight and balance data.
7) Systems Description - Describes operation in manner appropriate to pilot likely to fly that plane.
8) Handling, Service, and Maintenance - Recommended/Required inspections and maintenance
9) Supplements - Information necessary to safely operate aircraft with optional equipment, autopilot etc.
10) Safety Tips - (Optional) Cold weather operations, fuel conservation procedures etc.

Certificates & Documents

Prior to flying, we must ensure that not only we, but the aircraft is ready to go.

Training Requirements
For Recreational, Private, and Commercial Pilots, eligibility and experience requirements are found in Part 61 of the FARs.  Each certificate has its own privileges and limitations - also found in Part 61.

Medical Certificates
1st Class Medical - required for ATP certificate operations, expires at the end of the 6th month after exam.
2nd Class Medical - required for Commercial certificate operations, expires at end of 12th month after exam.
3rd Class Medical - required for Private Pilot operations.  Expires at end of 60th month (if under 40), or at end of 24th month if person is over 40.

Flight Instructors  - though paid and pilots (and would therefore assume a commercial operation), require only a 3rd class medical.

Recency of Experience
Every 24 months, a pilot must submit to a Flight Review to maintain their certificate privileges.  A practical test for another certificate may substitute for this review.

Carriage of Passengers - is only allowed if within the previous 90 days, the pilot has done 3 takeoffs and 3 landings in the same category and class of aircraft (and type if required).  If carrying passengers at night, the pilot must have (within the previous 90 days), done 3 takeoffs and 3 landings to a full stop.  The same applies to carriage of passengers in a tailwheel aircraft.

Required Entries in Logbook
Only required entries are those that are needed to demonstrate recency of experience for a certificate or rating, and flight review.

The following must be logged . . .
General - Date, Flight Time, and Departure/Arrival info.
Type of Experience/Training - Solo, PIC, SIC, flight/ground training, or Flight Simulator
Conditions - Day/Night, actual or simulated instrument, or flight simulator.

There are 4 documents a pilot must have in order to fly.

•Pilot Certificate
•Medical Certificate
•Photo ID
•Logbook Endorsements

See Airworthiness for certificates and documents required for aircraft.

Logbook Endorsements

Required Logbook Entries for Instruction Given


Sign logbook of each person given flight/ground training to and specify amount of instruction time and date. (FAR 61.189)

1) Date
2) Airplane Make and Model
3) Airplane ID number
4) Total time of flight
5) Place or points of departure and arrival
6) Type of pilot experience or training
7) Conditions of flight

Required Student Pilot Certificate Endorsements / Logbook Entries


•Student pilot certificate proveds space for solo / solo cross-country flight endorsement.
•Endorsement is for aircraft category (airplane), not make and model

Endorsement examples found at
http://faaflighttest.us/pilot_endorse.html

Preparation of a recommendation for pilot practical test / logbook entries


•Student and CFI must complete FAA Form 8710-1 (IACRA)
•New form is required for retest.

•Endorsement for flight proficiency must be in logbook.

Required Flight Instructor Records


•Required to maintain record in your flight instructor logbook or separate document containing:
 - name of each person whose logbook or student pilot certificate you endorsed for solo flight privileges, including type and date of each endorsement
 - name of each person you have signed off to take knowledge or practical test, including kind of test, date, and result of test

•Records must be retained for a period of 3 years.

Airspace

Airspace exists to protect something, whether that be certain aircraft at busy airports, or military operations areas.  There are two primary types of airspace - Controlled and Uncontrolled

Types of Controlled Airspace

Class A
Class B
Class C
Class D
Class E

Class A Airspace is primarily for high-altitude enroute traffic.  It exists at and above 18,000 ft MSL up to FL 600.  Requires instrument-rated aircraft with instrument-rated pilots on instrument flight plan.  Requires Mode C Transponder.

Class B exists around the busiest international airports like O'Hare, LAX, Atlanta, or JFK.  Its dimensions vary depending on surrounding airports or terrain features.  Its area is generally from the surface to 10,000 ft MSL.  An ATC clearance is required, as well as Mode C Transponder.

Class C exists around busy airports that aren't busy enough to justify a Class B designation.  They consist of an inner area and a shelf area.  Their dimensions may vary, but are typically surface to 4000 ft AGL inner area with an outer shelf area extending 10nm from 1200 to 4000 ft AGL.  Mode C transponder is required as well as two-way radio communications.

Class D exists around towered airports that are not busy enough commercially to justify a Class C rating.  Area is generally surface to 2500 ft AGL.  Two-way radio communications required.

Class E exists in all areas not covered by the above airspace.  It extends upward from the surface, from 700 ft. AGL or 1200 ft. AGL.  If it is not designated on a chart, it begins at 14,500 ft MSL. and continues up to 18,000 ft MSL.  It begins again above FL 600.

Uncontrolled Airspace

Class G Airspace is airspace that ATC has no authority or responsibility to control traffic.  Pilots are on their own for traffic separation and adherence to weather mins etc.  Its dimensions begin at surface and go up to base of overlying airspace (usually Class E).

Special Use Airspace

Protects an area where it is safer to confine an activity.  Certain limitations may exist for entering traffic or there may be no limitations at all.  From least restrictive to most restrictive they are, . . .

Controlled Firing Areas - These are not depicted on charts because they discontinue activities immediately if an aircraft is spotted in the vicinity.

Alert Areas - Shown on charts and begin with "A-   "  Contain a high volume of training aircraft or other aerial activity.  No special entry requirements, but extra vigilance in traffic avoidance should be maintained.

Military Operations Areas - have defined lateral and vertical limits to separate military training from IFR traffic.  If in use, nonparticipating IFR traffic may be cleared through or vectored around the area.  Aircraft participating are not subject to speed limits and may be flying at low level.

Restricted Areas - Similar to MOAs, but more hazardous because now there may be live ordnance.  IFR flights may still be authorized.  If Restricted area is not active, ATC may route traffic through it.  If it is active or not handed over to the FAA, ATC will vector traffic around.

Warning Areas - Essentially the same as a Restricted area, but extends 12NM outward from the coast.   May be over U.S. or international waters or both.  There aren't entry restrictions as in a restricted area, but extreme caution should be exercised - particularly when offshore, a pilot may inadvertently enter international airspace and face difficulty reentering.

Prohibited Areas - Entry is prohibited!  This would include areas around Cape Canaveral, White House and the National Mall or other areas where national security is absolutely necessary.

Other Airspace Areas

Local Airport Advisory (LAA) - Facilities provided at landing airport with communications and ASOS/AWOS.  FSS acts as tower in a way.  Recommended but not required.

Military Training Routes - Used by military.  No speed limit.  Begin with either IR (Instrument) or VR (Visual).  3 digits indicates above 1500 ft. AGL  4 digits indicates below 1500ft. AGL.

Temporary Flight Restrictions - FDC Notam issued.  Can exist around President's arrival on Air Force One.  Dimensions are 30nm for presidential TFR typically.  They may also exist to provide safe operating area for disaster relief operations, or to protect a sporting event.  Which games generate TFRs is not in any officially published document.  Use google and be familiar with the area in which you are flying.

Parachute Jump Aircraft Operations - are depicted on charts and published in the A/FD

Published VFR Routes - exist to transition or work around complex airspace.  An example would be Special Flight Rules through LAX.

TRSAs - Functions as controlled airspace similar to Class D around primary airport.  Participation is voluntary, but recommended.

National Security Areas - Pilots requested to avoid flying through these depicted areas.

Weather Requirements

The weather requirements are based upon the level of control that the controlling facility/airspace has.  Ultimately, it depends on how much time or visibility is needed to avoid other traffic.

FARs - Regulations/Publications

While the FAR book is not something you take into the bathroom for some light reading, it is necessary to have as a source for all things regulatory in nature.

While the FAA and its lawyers had to make sure and cover all their bases, reading the CFRs from cover to cover is not necessary.  It is, however, helpful to to pay attention to key sections of the book.

FAR/CFR Regulations


Availability - beyond buying the actual book at pilot shops or book stores, the information is easily accessed via google or at http://www.faa.gov/regulations_policies/faa_regulations/.

Part 1 - Definitions:  Defines the words used by the FAA in the following parts.  Serves to improve clarity of the regulations.

 Part 61 - Certification:  This section deals with how pilots receive their certificates and ratings.  It includes requirements that must be met to receive the certificate or rating and the privileges and limitations accompanying them.

Part 91 - General Operating and Flight Rules:  This section deals with how your flying operations can be conducted.  This includes requirements for VFR vs. IFR.  What maintenance requirements are necessary and who can do them.  Noise limits and restrictions on larger turbine and multiengine aircraft.

NTSB 830 - Addresses aircraft accidents.  It deals with requirements to report an accident, what constitutes an accident vs. an incident and definitions of the severity of it, as well as preservation of wreckage.

A/FD
Source of information pertaining to airports, communications data, nav facilities, and special notices/procedures.  Published every 8 weeks for each of the 7 different geographical areas of the U.S.

AIM
An encyclopedia of all things pertaining to flying such as navigation, airspace, air traffic control operations, etc.

FAA Advisory Circular (AC)
Disseminated to provide guidance on subject matter.  This information is not regulatory, but advisory.

Notices to Airmen (NOTAM)
Time-critical information that is either temporary or relevant after the publishing of the A/FD and chart information.  3 Categories.

NOTAM (D) - info on airport or runway closures, nav aid outages, radar service availability.  Distributed when gathering weather information in a standard briefing.

NOTAM (L) - info on taxiway closures, beacon outages, nonessential airport lighting outages.  This is distributed locally.

NOTAM (FDC) - Regulatory - changes in instrument approaches (DAs etc.), changes to aeronautical charts, TFRs (Temporary Flight Restrictions)

The Notices to Airmen Publication is issued every 28 days.  Once published, the NOTAM is no longer provided in weather briefings unless requested.

Practical Test Standards (PTS)
FARs specify which knowledge and skill must be demonstrated by the applicant.  Can be obtained at pilot shops or often in PDF form from the FAA website.  Verify the year it was published as items do change.

Pilot's Operating Handbook (POH)
FAA requires a POH or an FAA-approved flight manual.  These can be found in the aircraft itself or purchased at pilot shops or online.

High Altitude Operations

It is advantageous to fly at high altitudes because a plane is more efficient (consumes less fuel).  Also, weather can be avoided for the most part as you can fly above it.  There are certain oxygen requirements that must be met when flying at higher altitudes.

Regulations
If flying above 12,500 ft and up to 14,000 ft., the flight crew must use supplemental oxygen after 30 mins.
Above 14,000 ft., the flight crew must wear oxygen at all times.
Above 15,000 ft, the passengers must be provided with (not required to use) supplemental oxygen.

•In order to fly a pressurized aircraft that has a service ceiling or max. altitude above 25,000 ft. unless they have an endorsement proving they have received ground/flight training in that aircraft. (FAR 61.31)

•In an aircraft with a pressurized cabin, you may not fly at altitudes above FL 250 unless at least a 10-min. supply of oxygen is available for each occupant to use in the event that an emergency descent is needed due to loss of cabin pressure.

•Above FL 350, one pilot at the controls must be wearing supplemental oxygen that supplies oxygen at all times or automatically delivers oxygen above 14,000 ft cabin pressure altitude.  If one pilot is away from the controls, the one at the controls must wear an oxygen mask at all times.

•Aviation oxygen must be at least 99.5% oxygen.  Medical oxygen and industrial types are not allowed as they contain too much water, which could freeze and block the breathing lines.

Aeromedical
•Hypoxia is the primary risk for high-altitude operations.  Hypoxic Hypoxia is the altitude-specific variant - resulting from the overall lack of oxygen to the lungs, blood, and brain.  Other factors such as smoking, alcohol, or drug use can hamper the body's ability to function at altitude.

•Prolonged use of oxygen can also be harmful, as it is somewhat toxic.

•Nitrogen is inhaled when we breathe.  It is often naturally exhaled with carbon dioxide, but some gets absorbed by the body tissues.  If the surrounding atmospheric pressure lowers drastically (as in sudden decompression), the nitrogen could change from its liquid state and become a gas and form bubbles (the bends).

•Vision deteriorates with altitude

Operation of Pressurization Systems
Engine air is compressed by use of turbocharger or superchargers and may use a flow control venturi to control the amount entering the cabin.

An outflow valve is used to allow air inside the cabin to exit.  This allows a more constant stream of air into the cabin.

A common pressure altitude used in pressurized aircraft is 8,000 ft.  First, some more


TERMS


Aircraft altitude - actual height above Sea Level that aircraft is flying
Ambient Temperature - Temp. in area immediately surrounding aircraft
Ambient Pressure - pressure in area immediately surrounding aircraft
Cabin Altitude - Cabin pressure in terms of equivalent altitude above sea level.
Differential Pressure - Difference between cabin pressure and atmospheric pressure.

A Cabin Pressure Regulator controls cabin pressure to selected value and limits it to a preset in the differential range.  Differential control prevents excess pressure on the cabin from occurring.  When its max is reached, cabin altitude increases.

Cabin Air Pressure Safety Valve consists of pressure relief, vacuum relief, and dump valve. Pressure Relief prevents cabin pressure from exceeding differential pressures.  Vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing external air to enter cabin.  Dump Valve allows cabin air to be dumped to atmosphere.

Instruments include a Cabin Differential Pressure gauge to indicate difference between inside and outside pressure.

Cabin Altimeter provides a check on performance of system.

Cabin Rate of Climb shows rate in change of cabin pressure.




Decompression


Decompression is the inability of the system to maintain the desired pressure differential.  There are two kinds.


Explosive Decompression - a change in cabin pressure faster than the lungs can decompress (potentially causing lung damage).  Decompression that occurs in less than 0.5 seconds is considered explosive.
Rapid Decompression - Change in cabin pressure in which lungs are able to decompress faster than cabin.  No likelihood of lung damage.

In explosive decompression, the cabin may fill with fog, dust, debris.  Air rushes out of the body.

In rapid decompression, the period of useful consciousness is reduced because pressure on the body is reduced.  A pilot's effective performance is therefore reduced.

In decompression, prompt emergency descent is necessary to minimize hypoxia and other factors.

Types of Oxygen Systems


Cannula is a piece of plastic tubing similar in appearance to what an emphysema sufferer would wear, but these may only be used up to 18,000 ft.

Diluter-Demand supplies oxygen only during inhalation.  Is safe up to 40,000 feet.  Can supply either 100% oxygen or mix with cabin air.

Pressure-Demand supplies oxygen under pressure at cabin altitudes above 34,000 ft.  Users lungs can be pressurized with oxygen.  Can be used above 40,000 feet.

Continuous Flow are commonly provided for passengers.  They consist of a reservoir bag, which collects oxygen during time when user is exhaling.

Electrical Pulse-Demand deliver oxygen by detecting individual user's inhalation effort.  This reduces oxygen needed and is thus more efficient.

Pulse Oximeters detect amount of oxygen in blood by detecting color changes in red blood cells.

Oxygen should be stored in high pressure containers from 1800 to 2200 PSI.