Look out for Ice Ridges on the Lower Nose Fuselage

The crew of an A320 arrived at the aircraft to start a new day of flight early on a winter’s morning in Northern Europe. The ground temperature was reading -5°C and their aircraft was still covered with snow and ice from the overnight layover.

A two steps de-icing/anti-icing was performed before departure. Sprayed areas were the wings, vertical fin and horizontal stabilizers. The fuselage areas were not de-iced.

With the ground servicing complete, the flight crew proceeded to takeoff. At lift-off, the flight controls law reverted to alternate law and the AUTO FLT A/THR OFF ECAM caution triggered. 12 seconds later, the Flight Directors (FD), Characteristic Speeds, TLU function and Autopilot availability were also lost. The FD and SPD LIM red flags were displayed on both PFD (fig.1) and at the end of the ECAM take-off inhibition phase, when the aircraft reached 1500ft, three ECAM alerts were displayed:

• NAV ADR DISAGREE
• F/CTL ALTN LAW
• AUTO FLT RUD TRV LIM SYS

The flight crew identified an airspeed discrepancy issue and then compared PFD1, PFD2 and the standby speed indications with the ground speed on the navigation display. They proceeded to switch off ADR 1+3 and performed an in-flight turn-back with the ADR2 ON.

The analysis of the flight recorder’s data shows successive discrepancies during the takeoff roll and takeoff phase between ADR1 and ADR3 airspeeds. The ADR2 airspeed is not recorded in the DFDR.

For investigation purposes, the airspeed during the take-off was simulated based on an aerodynamic model of an A320 and using the recorded pitch and stick inputs from the event.

The resulting Computed Airspeed (CAS) from the simulation, representative of the actual airspeed is shown in blue on the graph (fig.2). This was compared to the recorded CAPT CAS (from ADR1) shown in red and ADR3 CAS, which is shown in green on the graph (fig.2). 

From the beginning of the take-off roll, ADR3 airspeed is perpetually underestimated up to 40kts and ADR1 airspeed is underestimated from take-off roll up to 10kts and from rotation up to 35kts. 

(fig.2) Comparison between recorded ADR1 & ADR3 airspeeds and the simulated airspeed

The investigation concluded that the root cause of such speed discrepancies was the build-up of ice ridges on the lower nose fuselage in front of the Pitot probes and lower nose fuselage not de-iced before departure. This creates airflow perturbations and causes airspeed computed value to be lower than the actual airspeed. 

The main cause of ice ridges over the lower nose fuselage of the aircraft is ice accretion during a long stay on ground in cold conditions (fig.3). A review of in-service events from the last 6 years shows that a large majority of ice ridges related events occurred during the first flight of the day.

Reported events also show that ice ridges may be dislodged during the flight or may remain attached to the lower nose fuselage for the entire flight (fig.4).

A second possible cause of reported ice ridge related events is when snow falling on a heated windshield melts and the water running down from the windshield refreezes in ridges on the lower fuselage. The caution note of the FCOM PRO-NOR-SUP- ADVERSE WEATHER – Ground Operations in Cold Weather Conditions describes this phenomenon.

The presence of ice ridges located forward of the Pitot probes on the lower nose fuselage creates airflow perturbations (fig.5) and may lead to airspeed data from the ADR of the impacted probe(s) to be lower than the actual airspeed. 

The effect of ice ridges on the measured airspeed value will depend on the location, shape and number of ice ridges present. A large ice ridge but also successive thin ice ridges can significantly impact the airspeed measurement. 

Regarding the effect of airflow perturbation caused by ice ridges, theoretically they could also affect the static ports or AOA sensors, but in-service data shows no effect on static pressures and rare effect on AOA measurements.

(fig.5) Effects of the ice ridges on the airflow forward of the Pitot probes

All “ice ridge” related in-service events that were reported to Airbus occurred on A320 family aircraft with the exception of one A330 event. However, we cannot rule out potential effects of ice ridges on the Multifunction Probes (MFP) installed on the A380 and A350 families, even if they are of a different design to the probes installed on other Airbus aircraft families. 

The presence of ice ridges in front of Pitot probes during flight can occur when the lower nose fuselage is not de-iced at all or not completely de-iced. This is why all personnel working to dispatch an aircraft, from maintenance staff to flight crew, should pay particular attention to the potential presence of ice ridges in cold weather conditions, especially for the first flight of the day or after an extended stay on ground. The lower nose fuselage must be clear of ice before departure to avoid unreliable airspeed situation due to ice ridges, and even thin ice ridges must be removed before departure.

Maintenance & De-Icing Crew:

The maintenance crew shall follow the guidelines in the AMM/MP Procedure 12-31-12 ICE & SNOW REMOVAL – MAINTENANCE PRACTICES to remove the snow and de-ice the aircraft.

On A320, A330 and A340 aircraft families, a dedicated AMM procedure 12-31-12-660-008-A – Forward Fuselage Ice Accretion De-Icing provides guidelines for removing ice and snow from the forward fuselage.

• While performing ice removal from lower nose fuselage it is recommended that:

The operator should spray the de-icing fluid from the rear to the front to avoid contaminating the Pitot tube

• Never spray de-icing fluid directly on Pitot probes, static probes and AOA probes to avoid contamination

More generally, AMM of all Airbus Aircraft types has been enhanced to highlight ice ridges phenomenon and to provide additional guidelines for de-icing operation. It will be also highlighted that while thin hoarfrost is permitted for example on the top surface of the fuselage, it must be distinguished from thin ice ridges that must be removed from lower nose fuselage.

Flight Crew:

The FCOM and FCTM of all Airbus aircraft were updated in 2018 to take into account the lessons learnt from these events.

• Modification of FCOM:

The FCOM (A320/A330/A340/A350/A380: PRO-NOR-SUP- ADVERSE WEATHER – Ground Operations in Cold Weather Conditions, A300/A310: Procedures and Techniques – Inclement Weather Operation – Aircraft Preparation for Cold Weather Operation) have been modified to explain that, during the exterior walkaround in cold weather conditions, the flight crew must check that there are no ice ridges on the lower nose fuselage, in front of the air probes. If ice ridges are detected, the flight crew must ask the de-icing personnel to remove them.

Lower nose fuselage check should be performed carefully because ice ridges can be difficult to see, especially on a white fuselage during night time

• FCTM modification:

The FCTM (A320/A330/A340/A350/A380: PR-NP-SP- ADVERSE WEATHER – Cold Weather Operations and Icing Conditions – General section, A300/A310: Supplementary Information – Inclement Weather Cold Weather Operations And Icing Conditions) was updated to explain the effect of ice ridges in front of Pitot probes on the airspeed measurement and the potential subsequent unreliable airspeed situation.

The means to prevent ice ridges described in this article can reduce the likelihood of unreliable airspeed events related to this phenomenon, however in any event where an airspeed discrepancy is detected by the flight crew, the UNRELIABLE SPEED INDICATION must be applied. Refer to FCOM UNRELIABLE SPEED INDICATION procedure and associated FCTM chapter for more information on the procedure application.