Transavia B737 at Rotterdam on Apr 24th 2021, we think we are 6500 feet, military radar tells FL110, unreliable speed and altitude on both left and right pitot systems

Last Update: March 23, 2023 / 09:23:59 GMT/Zulu time

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Incident Facts

Date of incident
Apr 24, 2021


Flight number

Alicante, Spain

Aircraft Registration

Aircraft Type
Boeing 737-700

ICAO Type Designator

A Transavia Boeing 737-700, registration PH-XRX performing flight HV-6051 from Rotterdam (Netherlands) to Alicante,SP (Spain), departed Rotterdam's runway 06 normally, however, on climb out the Mode-S transponder did not transmit any plausible altitude information. The crew soon after takeoff requested to level off, was cleared to maintain 5000 feet. Rotterdam ATC advised they did not receive any altitude information from the aircraft, the crew apparently switched source of the transponder and asked "and now", but ATC again advised still no altitude reading was available. ATC queried about their altitude, the crew reported they "believed" to be at 6500 feet, they were flying on their stand by instruments which they deemed reliable. ATC inquired with military three dimensional radar who informed the aircraft was actually at FL110. The crew decided to divert to Amsterdam and requested runway 06 at Schiphol Airport. On approach to Amsterdam the crew explained they had a normal takeoff from Rotterdam, in the climb they received indications of unreliable airspeed and altitude errors on both captain's and first officer's instruments, they believed their stand by instruments were reliable however. Amsterdam approach also confirmed receiving no altitude readout whatsoever. ATC and crew used another aircraft operating in the vicinity and maintaining FL070 to verify that the aircraft's TCAS indications were still working correctly, the crew reported they were descending through 4400 feet according to their stand by altimeter set to the local QNH, at the time the military radar reported they were at 3200 feet. The crew advised no assistance was needed after landing with the current situation on board. The aircraft landed safely on Amsterdam's runway 06 about 35 minutes after departure from Rotterdam.

A replacement Boeing 737-800 registration PH-HXC reached Alicante with a delay of 2.5 hours.

The occurrence aircraft is still on the ground in Amsterdam about 24 hours after landing. Prior to the occurrence it had last flown on Feb 19th 2021.

On May 11th 2021 the Dutch Safety Board (Onderzoeksraad, DSB) announced, they have opened a short/brief investigation on Apr 29th 2021 stating: "Shortly after takeoff, erroneous altitude and airspeed values were displayed on the captain’s and first officer’s instruments. The crew was able to reach adequate airspeed and altitude by use of standby instruments and decided to divert to Amsterdam Airport Schiphol, where the aircraft made a safe landing."

On Jun 3rd 2021 the DSB reported they are investigating two occurrences, this one as well as Incident: TUI Belgium B738 near Amsterdam on Oct 3rd 2020, unreliable airspeed, where unreliable airspeeds following storage due to the Corona Pandemic had occurred. As result of the investigations so far the DSB stated, that in one case the pitot covers had not been removed, in the other case the pitot pipes had not been properly reconnected, in both cases the crew were able to use visual references outside the aircraft in favourable weather conditions. The DSB stated that EASA well as Boeing had issued warning identifying safety risks of returning aircraft to service after long term storage. The DSB further stated that there is no possibility for flight crews to test the pitot systems prior to departure. The DSB subsequently argues, that the safety risk was known in both occurrences, yet, they did happen. The DSB anticipates that an increasing number of aircraft will be returned to service in the days and months ahead leading to an increase of numbers of non-standard maintenance. The incidents show, that extra attention is needed to address this risk. This is why the DSB issues an interim warning to airlines and maintenance companies of this safety risk.

In their quarterly bulletin for last quarter of 2022 issued on Mar 23rd 2023 the Onderzoeksraad (Dutch Safety Board DSB) stated the investigation is still ongoing but released the following conclusions:

The erroneous altitude and airspeed indications that manifested immediately after the aircraft had become airborne, were the result of multiple air data modules not receiving valid static air pressure inputs. Three pitot-static lines were not connected properly to their respective air data modules, because the connecting QDFs were likely not fully engaged on the lock pins or not connected at all after the flushing procedure was performed. This presented the flight crew with a hazardous situation in flight. The associated risk was reduced by the fact that they were flying in good weather conditions during the day and could make use of the aircraft’s standby instruments.

The operator had not designated the pitot-static system as a critical system and the associated critical tasks were therefore not performed at different times or by different technicians. The operator acknowledged the increased risks associated with maintenance action during the return to service procedures and addressed these by revising the associated task card. They issued a new task card after the incident flight, requiring two certifying technicians to independently verify completion of the tasks – in line with treating the pitot-static system as critical - and ensure the connections from the pitot-static system are properly connected. The operator also added notes to the task card to verify proper connection of the pitot-static lines to the air data modules and make sure that the actuation rings of the QDFs are fully engaged on the lock pins and the colored lock ring indicators are visible.

The DSB summarized the sequence of events:

On 24 April 2021, a Boeing 737-700 was scheduled for a flight from Rotterdam The Hague Airport (the Netherlands) to Alicante-Elche Airport (Spain). Prior to the flight, maintenance staff had performed a routine preflight inspection, including a visual inspection of the pitot-static system, and found no abnormalities. The pilots also performed their routine preflight checks and considered the aircraft in good condition. At 18.32 hours, the aircraft took off from Runway 06. During the takeoff roll, the indicated airspeed and altitude indication appeared normal.

After rotation and initial climb, the pilot monitoring did not observe a positive rate of climb on the vertical speed indication and the altitude indication on the Primary Flight Display (PFD). In daylight and visual meteorological conditions, both pilots observed the aircraft was positively climbing by visual reference to the ground through the cockpit windows and they subsequently maintained a pitch of approximately 10 degrees nose up. Around the same time, both pilots noticed a rapid reduction of the airspeed indication and a steady altitude indication of approximately 0 feet on their respective PFDs. Due to the reduction of indicated airspeed, the Ground Proximity Warning System (GPWS) generated a windshear warning, an AIRSPEED LOW alert sounded and the stick shaker system activated temporarily. The airspeed indications reduced to approximately 45 knots (minimum value) and the altitude indications remained at approximately 0 feet for the remainder of the flight.

About one minute after takeoff, the flight crew alerted air traffic control by declaring an urgency (PAN-PAN call). The pilot flying continued the climb in a northeasterly direction by visual outside reference and used the standby airspeed indicator and altimeter. The flight crew performed the Airspeed Unreliable non-normal checklist and subsequently determined only the standby airspeed indicator and altimeter to be reliable.

Four minutes after takeoff, at the request of Rotterdam Tower, the flight crew switched communications to Schiphol Approach. On several occasions during the flight, the approach controller provided the flight crew with groundspeed and altitude information, based on radar data. Initially, the controller did not have any altitude information, but at a later stage, he was provided with altitude information from the Dutch Military 3D radar system. The aircraft reached an altitude of approximately 11,000 feet.

After discussing the situation the crew decided to divert to Amsterdam Airport Schiphol. Schiphol Approach informed the flight crew that Runways 06 and 36R were available and the weather conditions to be CAVOK. The pilots descended the aircraft to 3,000 feet and were vectored towards Runway 06. The flight crew indicated they did not require assistance of emergency services after landing. During the approach, the pilots received another GPWS windshear warning at approximately 150 feet altitude. The pilots landed the aircraft 38 minutes after they took off from Rotterdam and taxied to the gate.

The DSB also published this analysis so far:

On 19 February 2021, the aircraft in question was placed into storage at Rotterdam The Hague Airport. As part of this, the pitot-static system was covered to prevent pitot tubes and lines from becoming blocked (for example by insects, dust or debris). In order to put the aircraft back into a serviceable condition after storage, pitot-, static-, and drain-lines that form part of the air data system had to be disconnected, flushed and reconnected.

Procedures for covering and flushing the pitot-static system were performed according to the relevant Boeing Aircraft Maintenance Manuals (AMM), by two certifying technicians and an apprentice.

The technicians who reconnected the pitot-static lines to the air data system stated that they believed all the AMM tasks and subtasks had been performed by themselves or the apprentice under their supervision. They did not recall who was tasked to connect the QDFs related to this incident. Due to the design of the QDFs, when properly installed, it is highly unlikely they would disconnect by themselves. It is therefore likely the three QDFs were not correctly connected and fully engaged on the lock pins or not connected at all after the flushing procedure. The certifying technician, responsible for verification of this procedure did not notice the improper connections.

The operator’s engineering department had created a task card, containing a list of AMM tasks to be performed for returning the aircraft into service. This task card is updated regularly according to manufacturer and operator requirements. The technicians performing the flushing tasks were familiar with this procedure on the task card. They had previously used a task card, which contained all the AMM tasks - related to the flushing - as separate items. The latest revision, which was used in this case, only contained a reference to the tasks in the AMM.

The AMM provided a detailed step-by-step guidance and reference to perform all the required flushing (sub) tasks. However, the technicians believed not all AMM tasks needed to be performed, due to the simplified presentation on the revised task card and therefor did not consult the AMM. This contributed to the QDFs not being connected correctly.

With three QDFs disconnected from the air data modules, these air data modules were not provided the required pitot-static air pressure inputs and therefore were unable to provide the pilots with airspeed and altitude information after rotation.

The pilots made use of the standby altitude and airspeed indicator, a procedure which is practiced during training in a flight simulator. Additionally, the pilots were able to maintain outside visual reference in CAVOK daylight conditions and the aircraft could be kept under control. The flight continued uneventfully to Amsterdam Airport Schiphol.
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Incident Facts

Date of incident
Apr 24, 2021


Flight number

Alicante, Spain

Aircraft Registration

Aircraft Type
Boeing 737-700

ICAO Type Designator

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