The Airspeed Indicator ASI

The Airspeed indicator ASI

Pictured above are two conventional airspeed indicators found in most light aircraft. The ASI like the Altimeter and VSI are what we call pressure instruments.
Pressure instruments measure the atmospheric pressure by using a pitot-static system. The pitot static system detects:

• Total Pressure - Static + Dynamic pressure which is also called pitot pressure, this is measured by a pitot probe pictured below from a cessna.
  
  

• Static Pressure - The static pressure gets measured individually by either a static vent/port usually located in the undisturbed air on the airframe, or a combined static port on the pitot probe itself. Pictured Below

So what happens now we have a measurment of Total pressure and Static Pressure?

What you see in knots on the ASI is a measurement of dynamic pressure.
Dynamic pressure is just the difference between Total pressure and Static pressure:

• Dynamic Pressure = Total Pressure - Static Pressure
  
  *Example - total pressure has a value of 10 and static pressure has a value of 6 then dynamic pressure would be 4
  
  

Now we have an understanding how it works lets look at the parts that make up the pitot static system and the ASI.

• Pitot System

As you can see with the above diagram the airflow enters the probe, from there it gets distributed to the ASI, VSI and Altimeter.

Things i didnt mention above is:

• Pitot Heat - This heats the probe in icing conditions, and prevents blockage due to ice buildup.
  
  
• Drain opening - This does exactly as it says on the tin, drain any unwanted water retained in the probe. Drainers on the PA-28 warrior are located bottom left just below the left hand seat, but this may vary on different aircraft.
  
  
• Alternate Static Source - If the main static port/vent gets blocked then this is here for backup.
  

some more.

Cumulonimbus Formation ( aka CB's )

What are Cumulonimbus Clouds what do they look like?

Cumulonimbus clouds are the ultimate in cumulus development. The base of the CB can stretch several miles across. Typically forming between 10,000 - 12,000ft the CB can reach its peak upto altitudes in excess of 50,000ft way up into the tropopause. Checkout the pics below:

A well developed cumulonimbus cloud can be charactorized by an anvil like top. The anvil is caused by strong winds at high altitudes as well as an inversion which lays above the CB caused by a rise in temperature above the tropopause. Hail and lightning can also be discharged from the anvil.

People always associate Cumulonimbus clouds with thunderstorms, this is true but not all CB clouds become thunderstorms.
Whether the CB is host to an active thunderstorm or just simply drifting in its own icolated innocence, they both carry the same severe aviation hazards. My advice is to stay well clear of any CB's that you may encounter.

What dangers can i expect when i enter a CB in an aircraft?

Flying into a Cumulonimbus is extremly dangerous. Listed below are some of the extreme conditions you are likey to expect on entry to a cumulonimbus:

• Turbulence - Vertical movement within a cumulonimbus can be in excess of 5000fpm. With the mixing of violent updrafts and downdrafts, windshear and severe turbulence can be expected within a CB resulting in G limits being exceeded, structural damage or even failure.

• Icing - Moderate to severe icing can be expected especially in the higher levels. The severe icing is due to the high concentration of supercooled water droplets causing clear ice formation resulting in loss of lift. ( pictured below a king air with wing ice buildup )
  
  
  
  
  
  
• Electrical Hazards - Aircraft that fly in the vicinity of CB's may experience disturbances with communications and navigation equipment. Disturbances are primarily due to the high concentration of static energy stored within the cloud. Aircraft may also encounter static discharge in the form of a lightning strike to the airframe.
  
  
• Precipitation - Hail, Snow, Sleet and Rain can all be expected from a cumulonimbus cloud and can all cause serious hazards at any stage of flight. ( Pictured below an airliner serverely damaged by hail, could you imagine what it would do to a c152 )
  
  
  
  
  
  
  Pictured below fully developed CB and charactoristics
  
  
  

What causes a Cumulonimbus Cloud?

Cumulonimbus are formed primarily on three factors being present:

• High moisture content in the air
• A parcel of unstable air
• A lifting force:
  
  
• Orographic lifting when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain e.g a hill.
  
• Thermal lifting/Insolation when the surface of the ground gets heated by the sun. Through conduction with the surface the parcel of air will rise as it is less dense than the surrounding air.
• Frontal lifting happens when a cold air mass moves against and under a warmer air mass forcing the warmer air to rise.

Typically formed along fronts due to the lifting force, as the warm moist parcel of air rises, it reaches the cooler air above. Because of the cooler surrounding air the water vapour condenses and realeses latent heat. When latent heat is released this warms the surrounding air causing further lifting. As the parcel of air continues to rise the water droplets continue to cool and form a mixture of supercooled water droplets and ice particles. When the updrafts are no longer able to suspend the ice particles and water droplets within the cloud gravity takes effect and they begin to fall. As they fall they produce a downward movement of air also known as a downdraft, these downdrafts then conflict with the upward movement of the air within the cloud causing serious static buildup and marks the mature stage of the cell.

What causes the static buildup?

It is said that static buildup is caused by charged ice particles. Due to the extreme energy within the cloud, the ice particles collide with each other creating positive and negative charged ice particles.

Pictured above is an active CB hosting a thunderstorm. Looking at the picture you can see the separation of static electricity. Once static charge is sufficient enough and becomes seperated with some regions holding a positive charge and others a negative, this is when lightning becomes likely.

How long do they last for and how do they dissipate?

A thunderstorm with 1 cell usually lasts no more than 20 - 30 minutes.
A severe thunderstorm with a number of active cells can vary and last several hours.

• Dissipating stage ( Pictured Below )

Even though the cloud is dissipating does not mean that it is safe!

• Severe weather, such as strong or gusty surface winds, heavy rain, hail, and tornadoes, usually is confined to the dissipating stage.

Strong cold, Gusty downdrafts dominate in the dissipating stage. As the rush of cold air crashes towards to ground it then gets deflected causing a series of strong gusty winds and LLWS ( low level windshear ) which is extremely dangerous to aircraft and has proved fatal in many cases. People may also refer to this as the gust front.

A downrush of cold concentrated, strong straight line winds blowing outward is also know as the Microburst. These winds can cause great damage to aircraft and are near fatal if encountered on final approach.

A dissipated cumulonimbus ( Pictured Below )

Pictured below a dissipated cumulonimbus cloud, leaving a mass of cumulus, stratocumulus and altocumulus.

Summary:

They will suck you in and spit you out! Avoid all cumulonimbus by at least 15-20 nm, if they are forecast on route be very very cautious. Many light aircraft do not have approved equipment e.g anti icing, weather radar to prevent any encounters with a cumulonimbus, my advice would be DO NOT GO. Its better been on the ground wanting to be in the sky than in the sky wanting to be on the ground, mark my words.

I will update this thread accordingly. This post is to give a brief, simple and user friendly understanding of cumulonimbus clouds and associated dangers to pilots.

Craig Middleton