pitot-static system and
pressure system provides the source of air pressure for the
- ALTIMETER (ALT)
- VERTICAL SPEED
- AIRSPEED INDICATOR
The pitot tube is
normally mounted on the leading edge of a wing. The pitot tube on an aircraft
used only for flight under Visual Flight Rules (VFR) may not be
heated to prevent icing. Aircraft to be used under Instrument Flight Rules
(IFR) are heated electrically, to prevent icing when operating in
visible moisture and cold temperatures. A switch in the cockpit controls Pitot
The static pressure port
is normally found on the side of the fuselage. On later model aircraft, an
alternate static source is provided inside the cockpit. The pilot can select the
internal static source if the outside source becomes clogged with ice.. When the
pilot selects the alternative source, the instruments relying on the static
pressure may operate slightly differently.
- The altimeter (ALT) may
indicate a higher-than-actual altitude.
- The vertical speed
indicator (VSI) will momentarily indicate a climb, then will settle back the
- The Airspeed Indicator (ASI)
will indicate greater-than-normal airspeed. Altimeter (ALT)
The two parts of
the system are
(1) the pitot tube and pressure line
(2) the static pressure system and lines.
Airspeed errors can occur
if either the Pitot tube or the Static vent become blocked.
Pitot tube can become blocked on
the ground, either by bugs, or ice. In this event the ASI will read zero. The
airspeed will not increase as it should during the takeoff.
In Flight Blockage of Pitot - When this first
happens, if the aircraft is in steady flight the pilot will not realize there is
a problem because the ice simply seals the pressure inside the Expandable
Capsule When the aircraft descends for landing however the pressure inside the
capsule will remain constant, whether the aircraft accelerates, or decelerates.
As the aircraft descends the static pressure will increase. This will squeeze
the expandable capsule. Thus, the indicated airspeed will decrease as the
aircraft descends regardless of the actual airspeed.
The opposite would happen if the
Blockage of Static Vent - As long as the
aircraft remains at the same altitude the Airspeed indicator works normally
since the correct static pressure is sealed in the case. When the aircraft
descends the static pressure in the case will be lower than it should be. Thus,
the airspeed will begin to read progressively higher as the aircraft descends.
If the aircraft climbs the airspeed will
begin to read progressively lower.
what a pilot
DO consider pitot heat and alternate
static sources as mandatory instruments for flight in actual instrument
DO conduct a careful preflight of all
DO keep your scan moving and identify
any instrument(s) that give you conflicting information.
DO identify the instrument or system
that is in error by determining what makes sense and what doesn't.
DO eliminate the offending instrument
or system from your attention. Carrying instrument covers (usually used by
diabolical CFIIs for partial-panel training) to cover the inoperative
instruments can keep them from becoming a constant source of distraction.
Remember, your scan will tend to pick up the abnormal.
DO remember that pitch plus power
equals performance. As long as you have vacuum instruments available, you
can keep the airplane level. Set the pitch and power for the performance you
want and trust that the airplane will deliver.
DO consider a backup altimeter for
serious instrument flying. Even the altimeter watches are good enough to keep
you out of the terrain. Sky diving altimeters are also useful as a backup in
DON'T allow your attention to become
fixated on any one instrument or system. If you can't figure out why an
instrument is giving you the reading it is, there's a good chance there's a
problem. Integrate the readings from all other instruments and determine which
instrument is lying to you. Eliminate it.