Bird or Hail Strikes on the Radome

An A350 aircraft was climbing towards its cruise altitude when the _SURV WXR 1 FAULT_ ECAM alert triggered just before reaching 12 000 ft. The flight crew pressed the WXR SYS 2 pushbutton of the SURV panel to switch to weather radar system 2. The _SURV WXR 2 FAULT_ ECAM alert triggered shortly after. The flight crew then performed several weather radar system switchovers with the same result during the climb. The aircraft eventually reached its cruising altitude of 35 000 ft. The _SURV WXR 1+2 FAULT_ ECAM alert triggered. The flight crew contacted their Operations Control Center and decided to perform an in-flight turnback. They initiated the turnback and started to descend. The flight crew then heard a loud noise, followed by strong aerodynamic noise, and observed indicated speed discrepancies. The following ECAM cautions subsequently triggered: _NAV ISIS SPD UNRELIABLE_ , _NAV AIR DATA REDUNDANCY LOST_ , and _NAV RNP AR CAPABILITY DOWNGRADED_. Three minutes after the collapse, the flight control system temporarily reverted to alternate law for 17 minutes. Normal law was then recovered and was maintained for the remainder of the descent. There was another reversion to alternate law that occurred at the beginning of the approach lasting almost two minutes, after which normal law was recovered and maintained until landing.

When the aircraft reached the gate, the ground crew observed that the nose radome had collapsed onto the radar (fig.1).

The aircraft technical logbook revealed that the aircraft had a bird strike on the left side of the radome one month prior to the event. The technical logbook stated that an inspection was performed in accordance with the MP A350-A-05-51-14-00001-282A-A – Inspection of the Aircraft after a Bird Strike and that traces of bird strike were found on the outer surface left side of the radome, but there was no damage detected. The logbook did not specifically mention if the inner surface of the radome was also inspected. Therefore, it is not possible to confirm if a complete inspection of the radome was performed.

Confirmation of the bird strike

A detailed examination of the collapsed radome by Airbus confirmed that there was a bird strike to the left side of the radome prior to the event where the radome collapsed. DNA of a hawk was found as well as some paint micro-cracking around the likely impact area.

The inner skin disbonded from the composite structure of the radome, and there was damage to the honeycomb structure near the location of the bird strike. This damage compromised the structural resilience of the radome, which resulted in the radome collapsing.

The Post Flight Reports (PFR) of the aircraft showed intermittent and repetitive _SURV WXR 1 FAULT_, _SURV WXR 2 FAULT_, and _SURV WXR 1+2 FAULT_ ECAM alerts triggered during the three previous flights with the associated “Drive Unit – WXR antenna” failure message. This failure message is triggered if the antenna fails to move to its commanded position during an antenna scan cycle. It may be the result of a failure of the drive unit of the antenna or of a mechanical blockage of the antenna. System tests were performed on the ground at the end of each of the three flights with no fault found. The condition of the inner skin that had disbonded from the radome structure was not detected during these ground checks.

The radome is an aerodynamic weatherproof fairing that protects the radar antenna. It is manufactured with materials that allow transmission and reception of the radar radio waves with minimal interference.

All radomes on Airbus commercial aircraft since the A300 are composed of a composite sandwich structure constructed of a honeycomb core located between internal and external skins (except A380 which has a double sandwich honeycomb core). These skins were previously manufactured from Glass, Kevlar & quartz. The latest radomes are manufactured from S2-glass® materials.

When impacted by hail, a bird strike, or other foreign objects, a sandwich composite structure deforms and then may return to its original shape with little to no damage visible on its external surface, but with potentially significant damage to its internal structure. (fig.4). In-service experience shows that there is often very little trace of the impact on the outer surface of the radome whereas the honeycomb core can be damaged and the radome inner skin can be disbonded around the impact zone.

If the damage due to bird or hail strikes is not detected on the ground, the damaged area will be subject to several flight cycles. The air trapped between the honeycomb and disbonded skin (around 1 bar on the ground) will tend to inflate during each flight and create a bubble where the skin is disbanded due to the lower ambient air pressure at altitude (around 0.2 bar at cruise altitude) (fig.5).

If a damage is not detected, depending on its size and location, it may impair the movement of the weather radar antenna and trigger the weather radar alerts (fig.6).

When the aircraft is back on the ground and the radome inner skin bubble deflates, it may no longer impair the movement of the antenna (fig.7). This can explain why there was no fault found during the troubleshooting test of the weather radar system on ground.

Flight crews have an important role to play in reporting and detecting damage due to bird or hail strikes.

In the case of an actual or suspected bird or hail strike during flight, and regardless of the location on the aircraft, the flight crew must make a logbook entry to report the event to maintenance personnel, so that they can perform the appropriate aircraft inspection.

The flight crew report should provide detailed information to aid in isolating any issues such as:

– At the time of the bird or hail strike event:

• Aircraft configuration (position of the landing gears and flight controls)
• Flight phase

– At the time of and after the bird or hail strike event:

• Any erroneous engine, radio, or navigation system behaviors
• Any smells noticed in the air conditioning system (burning smells or other odors)
• List of ECAM (EICAS for A220) alerts that triggered
• Description of any other system malfunctions during or after the event

Conducting a thorough exterior walkaround inspection of the aircraft is also an opportunity to detect any potential bird/hail or FOD related damage. Should traces of bird or hail strike or FOD be found on any aircraft part, the flight crew must inform maintenance personnel and make a logbook entry.

Careful inspection of the aircraft as per the AMM/MP/AMP after a reported bird or hail strike is essential, to check if the aircraft is safe to perform the next flight or if component repair or replacement is required.

In-service experience shows that it may be difficult to identify the signs of a bird or hail strike due to the fact that composite parts may return back to their original profiles after impact and this may mask or hide damage to the internal structure. The inspection must, therefore, be performed right after the bird or hail strike event to maximize the chance of findings.

High quality additional lighting should be used to perform the inspection if in low light conditions. The use of a grazing light (applying directional light near the surface and lighting it at a narrow angle) to accentuate the shadows of uneven areas can help to detect defects.

In the case of a bird or hail strike, the damage may not be limited to a single zone. Therefore, regardless of the location of the reported bird or hail strike (e.g. engine, fuselage, wing leading edges, etc.) the radome must always be inspected since it may also be affected by the strike.

In all cases after bird or hail strikes, in addition to the external inspection, it is mandatory to open and inspect the internal structure and surface of the radome for signs of delamination and disbonding, even if there is no trace on the external side of the radome. Typical signs of disbonding of the inner skin can be uneven surfaces or skin discoloration (fig.8). If any damage is detected, it must be checked that it is within the allowable damage limit provided in the SRM/ASR/ASRP to continue in service. If the damage is outside the allowable damage limits, then the radome must be repaired or replaced.

The documentation for A380 aircraft (with double sandwich honeycomb core structure), and for A220 aircraft (single sandwich honeycomb core structure but with a different manufacturing process), does not currently require a systematic inspection of the internal surface of the radome in the case of every reported bird or hail strike. An update is under evaluation of the need to harmonize the procedure with the other Airbus aircraft types.

At the time of the event described above, the A350 fault isolation task to be applied in the case of a fault message concerning the weather antenna drive requested the confirmation of the fault by performing a radar system test:

• If the system test failed, the task requested an inspection of the inner structure and surface of the radome.
• If the system test revealed no fault, no further action was requested. However, as per Aircraft Fault Isolation philosophy, after three or more occurrences of a fault, the full fault identification procedure must be done. This includes inspecting the inner structure and surface of the radome when there are three or more occurrences of a weather radar antenna fault message.

An update of the troubleshooting/aircraft fault isolation task linked to the weather antenna drive failure was launched for A320/A330/A340 and A350 aircraft. This is to take into account the conditions described in the above event causing the possible inflation at altitude, and deflation on the ground of the disbonded inner skin bubble of the radome. The procedure now requires a systematic inspection of the inner structure or surface of the radome to check for damage whenever there is a failure message related to the weather antenna or antenna drive.