ADR Disagree in the Climb out of Kuwait: Whose Airspeed Is Lying?
An A320 departs OKBK at 13:20 local in July: OAT 47 °C, QNH 0997 hPa, wind 340/15 kt, runway 33L. In the climb, passing FL180 with the autopilot engaged in CLB and CLB thrust, the ECAM shows NAV ADR 1 FAULT, followed by ADR 2/1 DISAGREE. The commander's PFD airspeed reads 35 kt higher than the first officer's and continues to rise steadily. Both altitude indications agree, and both vertical speed indications are normal. The standby airspeed indicator, which uses its own pitot and static sources, agrees with the first officer. The aircraft had been washed overnight and pitot covers were fitted and removed. A subsequent workshop inspection found polishing compound blocking the No. 1 pitot drain hole. The commander's instinct is to reduce speed because the aeroplane appears to be accelerating.
1. Explain the mechanism producing the commander's indication and predict what will happen in a descent.
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Diagnosis: pitot side, not static. Three clues fix it. Altitude and vertical speed agree on both sides, so both static sources are healthy. Only the commander's airspeed is drifting. And the standby - an independent pitot and static system - agrees with the first officer.
Mechanism. The No. 1 pitot has trapped total pressure and cannot vent through its blocked drain hole. The trapped pressure is fixed at the value corresponding to the true total pressure at the moment of blockage. As the aircraft climbs, true static pressure falls while the trapped pitot pressure cannot follow. The airspeed indicator computes pitot minus static, so that difference grows and the indicator over-reads progressively - 35 kt high and rising at FL180 is the exact signature. Had the drain hole been open, the trapped pressure would have leaked out, the differential would have collapsed, and the indicator would have fallen towards zero instead.
Descent prediction. The error reverses. As static pressure rises in the descent, the trapped pitot pressure becomes relatively low, the differential shrinks below the true value, and the indicator under-reads - eventually approaching zero while the aircraft is genuinely fast. That is the dangerous direction: the crew can believe they are slow while actually near VMO, with overspeed warnings that look like spurious faults.
2. State the immediate crew actions and explain how to establish which airspeed source can be trusted.
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Memory items first. Autopilot and autothrust off; set a known attitude and thrust for level flight; level off. The critical insight is that the automation was following the bad source: with the AP in CLB and CLB thrust, the aircraft was pitching to hold the false high target speed, so the true airspeed was decaying throughout the event. The first correction after disconnecting is therefore to lower the nose and set climb or cruise thrust for the actual weight and level, then check pitch and power against the tables.
Which source is right. Cross-check four independent things: the standby airspeed indicator, the first officer's ADR, GPS ground speed, and attitude/engine parameters. A GPS groundspeed consistent with the first officer's CAS plus a plausible wind - while the commander's CAS sits 35 kt higher - confirms ADR 1 is failed. If altitude and vertical speed are normal on both sides, only the pitot side is bad, which narrows it further.
Then. Deselect or isolate ADR 1 per the FCOM, declare the failure to ATC in plain language, and plan to land at the nearest suitable aerodrome. Do not fly an approach or a complex SID on the failed side: fly the first officer's PFD, brief the use of standby instruments for the approach, and remember that FMS position and fuel predictions may also be unreliable, so plan the diversion on raw data.
3. Why is this failure particularly hazardous at high altitude over Kuwait in July, and what would have happened if the crew had obeyed the false indication?
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Narrow margins. At FL180 with a high OAT, the gap between the true airspeed and the low-speed buffet is smaller than at low level, and the buffet onset speed itself varies with Mach and temperature. The pitch-and-power tables exist precisely because no displayed CAS can be trusted in this situation.
Obeying the false indication. If the commander had pitched up or reduced thrust to 'control' the apparent 35 kt over-speed, the real airspeed would have fallen further towards the stall. The crew would then have faced a stall warning while the captain's PFD showed a comfortable speed - a direct contradiction between warning and display. The rule is to believe the stall warning and the independent standbys, not the single suspect display.
The reverse trap. Once established in the descent, the same instrument would have under-read. A crew that trusts it would push the nose down to 'increase' a speed that was already high, walking into a genuine overspeed with possible structural consequences.
Environment. Kuwait in July adds a hot, dusty, high-density-altitude departure and a high true airspeed for a given CAS, so any manoeuvring margin is already reduced. The far more common failure mode in the Gulf is a blocked pitot from dust, sand or washing residue, so the pre-flight walk-around and cover discipline matter as much as the in-flight drill.