Unreliable Airspeed after a Blocked Pitot on climb-out from Salalah
You are PF in a twin-jet climbing through FL180 at 280 kt IAS, 0.74 M, autopilot engaged in VNAV, from Salalah (OOSA) to Muscat. The left primary flight display suddenly shows a rapidly increasing airspeed, then a red boxed IAS, while the standby ASI and the right PFD remain steady at 280 kt. The left altimeter simultaneously reads 400 ft higher than the right and standby. The autopilot disengages and the aircraft pitches up; the left side shows an overspeed aural warning while the right side shows no warning. OAT is −18 °C, SAT probe agrees. The captain's side is the left. The first officer is flying.
1. Diagnose the failure and explain why the left altimeter and left ASI disagree with the right side in the way they do.
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This is a blocked pitot on the captain's (left) side, most likely blocked by ice at the tube inlet with the drain hole also obstructed.
- A blocked pitot with a sealed static line behaves as follows: as the aircraft climbs, the trapped pressure stays constant while the true static pressure falls, so the IAS increases - exactly the rapid rise seen before the red boxed IAS. The overspeed warning on that side follows.
- The left altimeter reads 400 ft high because the left static source is also partially obstructed or has a leak/ice restriction: a trapped static pressure higher than ambient makes the altimeter over-read. The simultaneous ASI and altimeter errors on the same side point to a common pitot-static source problem, not two independent instrument faults.
- The right side and standby agree with each other and with the flight path, so they are the reliable sources. The aircraft did not actually accelerate; the pitch-up was the autopilot responding to the false high-speed signal before disconnecting.
Key discriminator: a blocked static port alone would make the ASI under-read in the climb and the altimeter freeze; a blocked pitot makes the ASI over-read. The altimeter offset confirms the static side is also compromised on that side.
2. State the immediate actions and the pitch/thrust strategy, and explain what instruments you would use to fly the aircraft.
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Immediate actions (memory items first, then QR H):
- Disengage the autopilot and autothrottle - already done by the disconnect, but confirm and do not re-engage.
- Set a known pitch and thrust attitude. Do not chase the unreliable indications. Use approximately the pitch attitude and N1/thrust setting for the current phase from the QRH 'Unreliable Airspeed' table (typically ~10° pitch and climb thrust for a climb at this mass).
- Cross-check the good side and standby. Fly on the first officer's PFD and the standby instruments; if the standby ASI is independent (separate pitot-static), it is the tie-breaker.
- Turn off the affected pitot heat cycling? No - keep pitot heat ON; the blockage may clear. Do not attempt to 'fix' it by pulling the wrong CB in flight.
- Declare and level off. Advise ATC of the unreliable airspeed, request a level block, and consider a diversion to a suitable airfield with long, dry runways.
Pitch/thrust strategy: target the pitch-and-power combination for the intended flight path, not an airspeed. If a climb is needed, use the QRH climb N1 and pitch; accept that the actual speed is unknown but bounded by the good side. Do not allow the false overspeed to tempt a power reduction that would put the aircraft near the stall.
Approach planning: plan a long, straight-in approach with a highish VREF margin (e.g. VREF + 20 kt) flown on the good side and standby, and brief a go-around on attitude and power if indications diverge again. Land at a runway with ample length and good braking action.