SAS B736 at Oslo on Mar 25th 2015, flare requires 4 times the usual force on elevator

Last Update: June 3, 2016 / 16:09:04 GMT/Zulu time

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Incident Facts

Date of incident
Mar 25, 2015

Classification
Report

Flight number
SK-4403

Destination
Oslo, Norway

Aircraft Registration
LN-RPA

Aircraft Type
Boeing 737-600

ICAO Type Designator
B736

A SAS Scandinavian Airlines Boeing 737-600, registration LN-RPA performing flight SK-4403 from Tromso to Oslo (Norway), had been treated with de-icing fluid prior to departure from Tromso and performed an uneventful departure and cruise at FL400 (ambient temperature at FL400 -55 degrees C). The aircraft was on a stabilised ILS approach to Oslo's runway 01R, the captain (54, ATPL) disconnected autothrottle and autopilot at 700 feet AGL and continued manually in calm winds and near optimal trim so that only minor adjustments were needed to maintain the glidepath. Nonetheless, the captain felt something was not correct with the elevator, while the ailerons felt normal. When the aircraft descended through 20 feet AGL the captain pulled to initiate the flare, however, the aircraft did not instantly react, the captain, according to flight data recorders, finally pulled 59kg/130lbs, more than 4 times the normal force, until the aircraft finally reacted and lifted the nose. The aircraft touched down and rolled out normally. While taxiing in the captain informed maintenance about the difficulties with the elevator control.

Norway's Statens Haverikommisjon for Transport (SHT) released their final report in Norwegian arguing, that the incident in itsself was not suited to investigate further. However, as there had been an increasing number of incidents where higher than normal forces were needed on the elevator, the SHT have opened a "theme investigation" which includes this case, a previous investigation conducted by the SHT, see Incident: Norwegian B738 at Kittila on Dec 26th 2012, unintentional steep climb on ILS approach as well as the flight data, recorded by Quick Access Recorders, from all Norwegian Air Shuttle, Norwegian Air Norway and Scandinavian Airlines Boeing 737 Next Generation aircraft, overall about 800,000 flights.

The SHT stated as conclusions:

The aircraft was fully stabilized just prior to flare for landing. Both the SHT and the commander believe, that even if the commander had not succeeded in breaking free the elevator, the situation would not have evolved into the risk of an aviation accident. Accordingly the SHT rated the occurrence an incident (not a serious incident).

It had been necessary to use in the order of 4 times greater force on the evelator control than normal. The SHT considers that the elevator system on the Boeing 737 must be fully working without the crew needing to intervene and break the elevators loose.

The SHT considers that the case of LN-RPA is likely to deepen concerns over this problem further, therefore a theme investigation has been introduced, where the SHT looks at partially blocked elevators on Boeing 737NG aircraft and possible relationships.

The SHT stated that three safety recommendations to assess the risks involved had been issued to Boeing with the report into the Norwegian B738 event in Kittila in 2012, aircraft registration LN-DYM. Boeing in their reply principally acknowledged the recommendations, however advised, they did not intend to perform a new risk assessment or to implement measures to safeguard the risks involved. The SHT has taken further steps with the FAA and EASA as result, however, has not yet received reply since April 2016.

The SHT stated: "SHT is aware that there has been a greater number of incidents involving Boeing 737 in recent years, where it has been necessary to use larger forces on the elevator controls than normal."

The SHT reported that during the night there had been heavy snowfall, there was still snow falling and the ambient temperature was -1 degrees C at the time of departure preparations. LN-RPA had been treated with 566 liters of liquid composed of 120 liters of type 1 de-icing fluid and 446 liters of heated water in a first step, then 126 liters of type 2 deicing fluid were applied, the procedure took 11 minutes, the aircraft departed without delay following the deicing procedure.

Following the landing maintenance logged following:

Elevator residue inspection and cleaning performed. … Found large amount of de-icing fluid on balance panels.

Elevator pitot-static system flushing performed. … Little water came out.

Control column power force test performed. … All forces to move the control column within limits.

The flight data recorder revealed the commander had pulled 58 daN (130 lbs/59 kg) of force during the flare until the aircraft reacted and maintained about that force throughout the landing flare until normal touch down. Normal force values, based on previous flights, would be 12 to 16 daN.

In November 2015 the Interstate MAK (Accident Investigation Commission), who are also solely responsible for aircraft airworthiness certification in the countries of their responsibility including Russia, had suspended the airworthiness certificate for all Boeing 737 family aircraft - which was ignored by Russia's Civil Aviation Authority - citing concerns of constructive deficiencies in the 737 elevator system, see News: Russia suspends airworthiness certification for Boeing 737s, but does not prohibit operation of 737s. The MAK stated, that they had invoked a separate investigation into this issue over concerns, this issue might have had a role in the crash in Kazan, see Crash: Tatarstan B735 at Kazan on Nov 17th 2013, crashed on go-around and stated, they had not received satisfactory replies from Boeing and the FAA over the questions they raised during that investigation. This MAK investigation may become more volatile in nature by the crash in Rostov, see Crash: Flydubai B738 at Rostov on Don on Mar 19th 2016, lost height on go around after stabilizer moved nose down following holding for 2 hours.
Incident Facts

Date of incident
Mar 25, 2015

Classification
Report

Flight number
SK-4403

Destination
Oslo, Norway

Aircraft Registration
LN-RPA

Aircraft Type
Boeing 737-600

ICAO Type Designator
B736

This article is published under license from Avherald.com. © of text by Avherald.com.
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