Description

On 2 January 2014, an ATR 72-212A (EI-REL) operating a scheduled passenger flight from Manchester to Cork at night in Visual Meteorological Conditions (VMC) made a go around from its first approach due to the aircraft becoming unstable in turbulent conditions. During radar positioning for a second approach, forward visibility was obscured by the accretion of what were suspected at the time to be salt particles and a second go around was flown. After part of the windscreen had become transparent again, a third approach was successfully flown to an uneventful landing.

Investigation

The event was Investigated by the Irish AAIU. It was established that surface wind direction at Cork had been generally around runway heading with a mean speed of around 30 knots and some gusts as high as 50 knots. Warnings of severe turbulence and low level windshear were in force.

The aircraft commander was PF for the flight from Manchester. The aircraft carried out a go-around from its first VHF Omnidirectional Radio Range (VOR) approach to RWY 25 at EICK, due to a substantial increase in indicated airspeed on short finals which resulted in the approach becoming unstable. There were no issues with forward visibility during this first approach. After maintaining runway heading to 3000 feet as instructed by ATC and upon advising that they wished to make another approach, radar vectors for a left hand pattern at 3000 feet back to another VHF Omnidirectional Radio Range (VOR) approach were given. The track followed is reproduced on the illustration below. The downwind track south of the airport brought the aircraft close to the coast and at times over the sea.

The aircraft was cleared to land in similar wind conditions to those of the first approach. However, the aircraft subsequently carried out a go-around from its second approach and advised that the reason was a complete loss of forward visibility for the pilots because the windscreen was obscured by a thick residue which had adhered to the windscreens. ATC were advised that the windscreen wipers had not removed the deposits which appeared to have sealed and dried on the forward surfaces. The aircraft was instructed to climb to 4000 feet and initially given a radar heading of 330° because of an inbound Airbus AIRBUS A-321 positioning for an ILS approach to runway 17. The commander made a mobile phone call to the Operator's engineering office at Cork to discuss the problem and was informed that another company aircraft which had landed earlier in the evening had sustained some salt accretion which had reduced but not eliminated forward visibility. As a weather cell appeared to be on track, the commander requested ATC to continue the present radar heading in the hope that entry into precipitation might remove some or all of the contamination which was still preventing any forward visibility. ATC informed the crew that they were aware of a recent similar situation at Shannon in high wind conditions which turned out to have been caused by sea salt accretion.

Some exposure to precipitation in the shower activity resulted in clearance of a small area at the base of the commander's windscreen. ATC advised that the A321 had just landed and had believed that a film on the outside of their windscreen was sea salt. ATC advised that they could see the ATR 72 and had put all the runway lighting up to maximum and asked if the crew could see the airport to which an affirmative response was received accompanied by a request for a descent towards a further VOR approach to runway 25. The commander declared a PAN "in the light of the condition of the windscreen" and subsequently requested an Rescue and Fire Fighting Services Standby. Whilst on final approach, the aircraft entered a rain shower which further cleared the contamination from the windscreen and an uneventful landing followed.

The commander provided a comprehensive description of events to the Investigation, describing the effect of the salt deposits as "like a frosted glass windscreen". The contamination on the external surface which built up during the easterly track when being vectored for the second approach was described as so dry that the windscreen wipers had no effect on it. It was stressed that during the first approach and the initial go around, the windscreens had been clear. Prior to and as the salt deposits had originally begun to build up, it was confirmed that the aircraft had been flying through clear air and that there had been no evidence of ice on the windscreen. It was suspected that the effect of the windscreen heating may have been to accelerate the drying of the salt deposits. It was reported that, at the time the small area of the left hand forward windscreen (which was just "3 inches wide and 1 inch high") cleared, there had been no similar clearance on the right hand forward windscreen.

The commander of the other Company ATR 72 aircraft which had also been affected by salt contamination, but to a much lesser extent, advised the Investigation that having approached the airport from the north, their aircraft had not flown over the sea or in the vicinity of the coastline. Flying in VMC, salt accretion had only been noticed when on right base for a VOR final to runway 25 about two minutes before the right turn into finals. The perception was that the build up was relatively rapid and it had the effect of blurring forward vision.

The A321 crew had not flown over the sea below 8000 feet and did not notice any loss of visibility on their approach but once under apron lighting as they taxied in, they had noticed a powdery, dusty substance adhering to their forward windscreens that "you could write your name on" and which they later told the TWR they believed had been sea salt.

The Investigation noted that the ATR 72 does not have windscreen washers. Windscreen heating keeps the outer surface above 2°C and the inner one above 21°C (to prevent mist formation).

During the course of the Investigation, the Operator involved advised experiencing another case of reduced forward visibility due to windscreen salt accretion in January 2015 when an aircraft which had been unable to land off two turbulent daylight approaches in high winds at Dublin had begun a diversion to Belfast Aldergrove with a light layer of salt on both forward windscreens which had got worse en route to the extent that the commander was only able to see ahead to land by raising his seat so that he could look through a small clear area at the very top of his windscreen.

A check with both Airbus and ATR found that they had no record of any salt accretion events, but the Investigation did find one case where sea salt accretion in similar conditions to those prevailing in the event being investigated had led to the successive transient failure of three out the four engines on a Lockheed Orion being operated by the US National Oceanic and Atmospheric Administration (NOAA) in February 2007. The Report on the investigation into that event documented a number of factors which it was concluded could contribute to the generation of a dangerous operating environment due to the presence of a high atmospheric concentration of sea salt aerosol:

• A large difference between the sea surface temperature and the air temperature, particularly with warm water and cold air. Large horizontal temperature gradients within the ocean also appear to contribute significantly.

• High surface wind speeds.

• Lack of precipitation, particularly in ambient air temperatures near 0°C.

• Relative humidity at or above 80%.

• The height of the marine boundary layer - the high salt environment will not extend above a well-defined boundary layer.

It was noted that this Report also concludes that "precipitation will very effectively remove salt particles from the atmosphere, even in the presence of large vertical mixing velocities" and that "dangerous concentrations of sea salt aerosol are not anticipated above 5000 feet due to dilution and settling".

After their event, it was found that NOAA had asked the Marine Meteorology Division of the US Naval Research Laboratory (NRL) about the risk of further encounters with the conditions experienced and the Laboratory had subsequently published a Report on the subject. This concluded that high concentrations of airborne sea salt particles can result when there is little precipitation over the sea and wind speeds exceed 50 knots and especially when they exceed 60 knots. The report considered it likely that salt particles with diameters greater than 15 µm can reach over 2500 feet and under the worst conditions probably 5000 feet and concluded that "the high-wind, dry, cold sector behind a front or to the southwest of an occluded system appears to be the most dangerous region in the storm for sea salt fouling".

The Irish Meteorological Service were consulted on the subject at the centre of the Investigation and stated that "their research (had) concluded that sea salt windscreen accretion on aircraft was a very rare phenomenon". However, given that the Service's aftercast for the subject event stated that an occluded front had recently cleared the area, the similarity to the conditions mentioned in the NRL Report was evident. The Investigation also noted that since the effectiveness of precipitation in scavenging salt particles from the atmosphere is widely accepted, the general, although not complete, absence of precipitation in the Cork area at the time of the investigated event was in line with an expectation of high levels of airborne salt particles.

The Investigation formally determined that the Probable Cause of the occurrence was "loss of forward visibility due to sea salt accretion on the front windscreens".

It was also determined that a Contributory Factor had been "a confluence of meteorological circumstances including high marine surface winds combined with a lack of precipitation".

The Investigation issued a reminder to aircraft operators that in the event of an aircraft operating in areas of high concentrations of sea salt aerosol, particular attention should be paid to the washing of engines and airframe as a corrosion prevention measure.

Safety Action

The Operator issued a Flight Crew Instruction on 'Sea Salt Aerosol Accretion' which contained general information on the generation of sea salt contamination, similar to the information contained in this Report. It reminded crews that, in the event of experiencing contamination leading to degradation of visibility through the windscreen of an aircraft, it may be necessary to fly through precipitation prior to conducting a landing.

The Final Report was published on 30 April 2015. It was considered that, given the rarity of this type of event, no specific Safety Recommendations were warranted and that a general raising of awareness through the publication of the findings of the Investigation would be sufficient.

Further Reading

• Missed Approach
• Low Level Wind Shear
• Turbulence
• Situational Awareness
• Emergency Communications
• NOAA Lockheed Orion Mishap Report
• US Navy NRL Report, October 25, 2007