The following events may cause the air data to become invalid:
On GND If at 80kts airspeed indications are substantially different, the airplane should be considered unsafe to fly and takeoff should be aborted. Sound judgment or remaining runway length is required, because 80 kts may not be 80kts.
Verify ground speed indication!
If the pitot system is covered or blocked, airspeed indication may be zero or showing low. If static ports are covered or taped, airspeed shows normal during takeoff ground run. However, airspeed and altitude become invalid when airborne.
In FLT FCTM 9.x [Airspeed Unreliable]
- Maintain your actual body attitude and power setting!
- Determine the invalid air data source!
Invalid airspeed data may cause following systems to trigger a false warning :
A false high airspeed indication may cause the Flap Load Relief system to inhibit flaps extension beyond F25/F30 or even F10 (with SFP). Use Alternate Flaps extension to bypass the load limiter, if flaps 40 is desired.
If the radome is missing or severely damaged, antenna's for G/S, LOC and WXR may have damage too and may be unserviceable or erroneous! Choose a landing runway that allows for a LNAV/VNAV, non-ILS or visual approach according to actual weather.
Basic Rule : PERFORMANCE = THRUST x ATTITUDE
Proper combination of thrust (engine %N1) and attitude (pitch degrees) always results in the desired performance! Knowledge of airplane thrust settings and pitch attitudes for various flight maneuvers can help identify airspeed anomalies. Refer to PI [General Flight with Unreliable Airspeed] for %N1 and pitch attitudes.
Do not allow false airspeed, altitudes, vertical speed or false aural warnings to confuse or stress you! Incorrect recognition of the faulty indications and delay in corrective actions can result in loss of airplane control!
Ground Speed, Radio Altimeter and GPS altitude (if available) are the only reliable resources with a damaged or blocked pitot/static system or an air data source failure.
Rule of thumb :
Above FL270, maintain a GS (zero wind) of 450 kts
Below FL270, use the following formula to obtain your actual IAS :
IAS = TAS - FL / 2
e.g. GS shows 350 kts at FL100, the IAS is 300 kts
Do NOT use wind indicated on the ND or the FMC! Use wind predictions from your flight log (which you may have inserted into the LEGS page) or ask for a spot wind reading from a nearby aircraft.
Provided that the radome and vanes are not damaged, the AoA gauge also remains reliable, except for the red stick shaker line. Maintain a safe AoA between 0 degrees and 12 degrees(typical 6 degrees for approach). If an AoA gauge is not installed, use AoA = Body Attitude - FPA (no wind). Speedtape amber and red bands and also PLI may also be unreliable!
ALTITUDE UNRELIABLE
Altimeter, Vertical Speed and FPV indications are also questionable, if any of the static ports is blocked. Do not rely on ATC transponder altitude readings, but ground speed measured by ATC is reliable. EGPWS terrain display is using not only GPS but also ADIRU altitude inputs, and may therefore be erroneous.
An invalid auxiliary static air signal will cause an invalid cabin differential pressure indication and may disturb pressurization control.
If you remain uncertain about your altitude and your estimate it to be below 15,000 feet, consider depressurizing the airplane and use the CABIN ALT indication as altimeter. The CABIN ALT measures pressure inside the airplane.
Weather and fuel permitting, consider diverting to an airport near the coast line and use your also altimeter to descent over the water.
Another method to define an approximate altitude is to turn the WX radar tilt 6 degrees down ; the distance to first ground return equals your height above terrain in thousands of feet.
AIRSPEED AND ALTITUDE UNRELIABLE
If you remain uncertain about airspeed and altitude, do not hesitate to ask for a formation flight with any nearby civil or military aircraft for approach and landing !
