Airspeed indicator

At approach for takeoff (i.e. Vj to Vr to V2), the sensed dynamic pressure=0, so airspeed indicator remains pegged at its lower stop... 

However, at takeoff, the airspeed indicator will suddenly come to life because the static pressure starts to drop. The indicated airspeed continues to increase through the correct value as the aeroplane climbs, but continues until the can appear to be exceeded, triggering an overspeed warning. 

If the pilot trasts the faulty airspeed indicator, there is grave danger of increasing pitch or reducing thrust or both to reduce the erroneous indicated airspeed. This could result in an aircraft stall. 

During takeoff, both the altimeter and the airspeed indicator operate correctly. 

The sensed dynamic pressure (i.e. indicated airspeed) fails to increase as rapidly as it should during climh. If the aircraft actually climbs at a constant speed, the airspeed indication decays, eventually reaching near zero. 

If the captain rehes on the airspeed indicator, the typical response will be to reduce the pitch attitude to maintain the erroneous airspeed, possibly causing the aircraft to exceed its airspeed limitations. Overspeed warning may not operate if connected to the same erroneous airspeed source. 

Airspeed Indicator Aircraft s speedometer, giving speed based on the difference between ram and static air pressure. 

Flight Instruments. Flight instruments operate using either gyroscopes or air pressure. The instruments that use air pressure are the altimeter, the vertical speed indicator, and the airspeed indicator. Airplanes are fitted with two pressure sensors the pitot tube, which is mounted under a wing or the front... 

Differential pressure flowmeters are suited to high- and moderate-velocity flow of gas and clean, low-viscosity liquids. Venturi flowmeters (Fig. 18.9(a)) are the most accurate, but they are large and expensive. Orifice flowmeters (Fig. 18.9(b)) aresmaUer, less expensive, and much less accurate than venturi flowmeters. Nozzle flowmeters (Fig. 18.9(c)) are a compromise between venturi and orifice flowmeters. Pipe-bend flowmeters (Fig. 18.9(d)), which can essentially be installed in any bend in an existing piping system, are used primarily for gross flow rate measurements. Pitot-static flowmeters (Fig. 18.9(e)) are used in flows which have a large cross-sectional area, such as in wind tunnels. Pitot-static flowmeters are also used in freestream applications such as airspeed indicators for aircraft. 

A number of flight instruments (e.g. airspeed indicator, altimeter, horizontal situation indicator) are to be displayed on a single screen. 

All three accidents involved aircraft with digital (computerized) cockpits, where the pilots received all their information and alarms about the state of the aircraft from computer displays. All three aircraft suffered blocked Pitot tubes which led to erroneous airspeed indications. In each case the pilots lost situation awareness for a critical short period of time, and aircraft that were otherwise in perfect flying condition crashed with the loss of all passengers and crew. 

As the plane was accelerating in darkness for takeoff at 2342 26 h local time. Captain Erdem saw that his airspeed indicator (ASl) was not working. 

Five sources of velocity information were available to the crew. They included Captain Erdem s airspeed indicator, the First Officer Gergin s airspeed indicator, a standby airspeed indicator in the center of the instrument panel, a groundspeed readout on Captain Erdem s Electronic Right Information System (ERS) display, and a groundspeed readout on First Officer Gergin s display. 

At 2343 00, Captain Erdem said It began to operate , meaning that his airspeed indication was working again. 

After wheels-up, the autopilot was engaged and the climb continued normally. Unfortunately, the autopilot was selected to use Erdem s (faulty) airspeed indicator. 

The Captain said, Both of them are wrong. What can we do followed by Alternate is correct , presumably meaning that the standby airspeed indicator in the center of the instrument panel was working properly. 

At 2345 52 the stick-shaker began to operate and continued until the crash. The stick-shaker is a device used to teU the pilots that they are close to stall speed - the control column is made to vibrate as an inescapable warning of low speed. The aircraft was at 7132 feet and Erdem s faulty airspeed indicator was showing 323 knots, when the fine speed was less than 200 knots. 

The US National Transportation Safety Board (NTSB) issued various Safety Recommendations on May 31, 1996. These included a recommendation that the Boeing 757 flight manual should be revised to notify pilots that Simultaneous activation of the mach speed trim and rudder ratio advisories is an indication of an airspeed discrepancy. The NTSB also required Boeing to modify the alarm system to include a caution alert when an erroneous airspeed indication is selected. Various other changes to the flight manual were also instructed. Simulator training... 

The two flight crew were now over the ocean and trying to fly the aircraft manually to return to Lima in darkness, all the time with abnormal or non-functioning altitude and airspeed indications, and with the Electronic Flight Information System generating lots of alarms. 

What had actually happened was, however, far more prosaic. Debris recovered from the seabed showed that the Pitot tubes (used for airspeed indication) and also the static pressure ports (used for barometric altitude measurement) had been covered by masking tape. This tape was used when the aircraft was polished. Quahty control checks should have taken place to confirm the tape was removed - a (unnamed) duty supervisor and line chief were responsible. One of the pilots should also have carried out visual checks as part of pre-flight checks. 

There was a noise interpreted as ice crystals hitting the plane, and shortly after there was an alarm indicating that the autopilot has disconnected. This was caused by the Pitot tubes icing over. Temporarily, the pilots had lost all airspeed indications. This should not have been a problem - other pilots have flown simulations where they have been able to continue quite safely. However, neither Bonin nor Robert had received training in dealing with loss of indicated airspeed at high altitude, or in flying the plane in such conditions. 

Flight Safety Foundation, Erroneous airspeed indications cited in Boeing 757 control loss. Accident Prevention, vol. 56 No. 10, October 1999. 

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