Yemenia Flight 626 Detailed Meteorological Analysis

Tuesday, November 17, 2009
By George Hatcher

by Tim Vasquez
posted with permission

Flight details
Flight 626 departed Sana’a, Yemen (IATA SAH, ICAO OYSN) at 29/1830 UTC (29/2130L) on a four-hour flight destined for Moroni, Comoris Islands (Prince Said Ibrahim International Airport, also known as Hahaya Airport, IATA HAH, ICAO FMCH, WMO 67002. The crash occurred at approximately 29/2250 UTC (30/0150L). The moon phase was a waxing half moon which set at 29/2123 UTC (30/0023L) in Comoros. Therefore there was no solar or lunar illumination available for visual reference, and this does raise the possibility of disorientation as a factor.

Position of crash
So far, all available information has come from press reports. Gen Bruno de Bourdoncle de Saint-Salvy, French naval commander in the Indian Ocean, said the plane came down about 15 km (eight nautical miles) north of the Comoran coast (source). Another report indicates the crash occurred as the plane approached the airport; the plane “tried to land, but couldn’t, and then U-turned before it crashed” (source). This gives a tentative position of roughly 11°14’S 43°19’E.

Presumptions in this incident
The profile of the approach and the aircraft track is not known due to incomplete media reports and conflicting details about a missed approach. It is significant that Runway 20 would have been the active runway due to the strong southerly winds, requiring a nighttime VFR approach, however there are no published approaches for Runway 20 (source). The location north or northwest of the airport indicates the aircraft was not on a critical leg of its approach and probably was maneuvering at some altitude (3000 to 10,000 ft MSL) to establish itself on the approach.

Meteorological analysis

Surface analysis
The surface analysis showed that the Comoros Islands was not in a typical trade wind regime driven by subtropical high pressure but was under the influence of a decaying polar front generated by a polar high that had spread into South Africa. At approximately 29 June / 0600Z, 17 hours before the incident, a decaying cold front arrived at Moroni with significant strengthening of winds and a shift from southeasterly to southwesterly. The air mass driving this front had been responsible for hard freezes in Zimbabwe Sunday and Monday (source). Though polar fronts are normally found only in temperate latitudes, their presence in tropical locations has been well-documented in the literature.

This shows that the strong winds are persistent, spanning many hours, and that cloud cover was generally less than 1/8th (mostly clear) at Moroni.
Sea surface temperatures around the Comoros Islands were 26-27 deg C (79-81 deg F). The strong winds sustained to 29 mph, according to the basic Beaufort Scale definition, are associated with waves of 9 to 13 ft and this would be supported by the long southerly fetch from the Mozambique Channel. Preliminary model data from Oceanweather and FNMOC indicates significant wave heights around Comoros were on the order of 3 to 4.5 meters (10 to 15 ft), which would have caused significant problems for ditching, especially without visual reference.

Satellite imagery
At the time of this writing, only 4 km infrared imagery was available and no calibrated spectral data. The 4 km imagery showed a quiescent weather pattern across the Comoros Islands with no deep convection. Layers of broken stratocumulus or altocumulus were prevalent throughout the Mozambique Channel. Due to limited resolution there was insufficient data to determine the extent and coverage of stratocumulus or stratus layers.

Strong winds

Yemeni civil aviation deputy chief Mohammed Abdul Qader said the wind speed was 61 km/h (38 mph) as the aircraft was landing (source). This is consistent with observed METAR reports. While these winds can significantly complicate approaches and takeoffs, a high wind speed in itself is ground-relative and does not affect on other phases of flight except for (1) causing course deviations off of presumed routes due to drift and (2) causing mechanical turbulence and shear especially near rough terrain.

Scott Bachmeier of the University of Wisconsin Space Science and Engineering Center was kind enough to assist with imagery, and contributed these additional thoughts: “The Meteosat-7 IR cloud tracked winds certainly show the strong equatorward flow (~20-30 knots) that likely caused problems on the approach to the airport on the Comoros Island.”

Grande Comore Island, where Moroni is located, is a mountainous island about 10 x 30 miles in size with volcanic peaks to 2361 m (7746 ft) MSL that include Karthala, an active volcano. As a result, the island does significantly disrupt the lower tropospheric wind flow in a strong wind regime such as this one, and probably shares characteristics with similarly-dimensioned islands like Kauai, where damaging turbulence incidents have occurred in the past. AQUA high-resolution imagery the next day indicated convergence line downstream from Grande Comodore and the presence of mesoscale open cell convection (see Agee et al. 1973 for more information) which is typical of a cool air mass over a warm surface. While this does not propose that any circulations existed which damaged the plane, it does highlight that the winds were likely variable and somewhat unpredictable, and some shear and turbulence may have existed in the lowest several thousand feet of the atmosphere.

Conclusions

Once again we are left with another Airbus mystery, and while it appears certain that weather was not the cause, the weather conditions across the Comoros Islands were atypical and there is some possibility it could have been a contributing factor to the crash. Some individual conclusions are as follows.

  • Conditions enroute. Naturally, consideration must be given to whether damage may have occurred enroute from Yemen and manifested itself during the approach. A review of the satellite imagery showed most of the flight was completely free of weather. However a few strong cumulonimbus clouds (thunderstorms) were present in the Sana’a area at the time of the flight’s takeoff. Sana’a reported no significant weather during the evening except for scattered cumulus and stratocumulus at 3000 ft most of the night, but satellite imagery (here) indicated thunderstorms in the area with -60 deg C tops, probably which were largely confined to orographically-driven storms on the mountains just southwest of Sana’a. These would have been decaying with the loss of solar heating. Until I get archival-grade satellite datasets, I cannot say how close the storms were to Sana’a International Airport or whether they may have been on the departure path, but it does appear they were within about 20 miles of the airfield.
  • Precipitation. The atmosphere at Comoros was far too stable to support deep convection (including thunderstorms). The cold air advection over warm ocean waters did produce sufficient instability for shallow convection, and large water droplets could have existed in these clouds.
  • Icing. Icing was not a factor. The freezing level was above FL150 with extremely dry air through at least FL350.
  • Turbulence. Light to moderate low-level turbulence is likely due to mechanical turbulence from the strong surface winds and cold air advection in the presence of diabatic heating over the warm ocean waters. This could have been compounded further by perturbations embedded in the wake flow downwind of Grande Comodore Island, where the flight is believed to have crashed. As the temperature difference between the water and the air mass was not substantial, it is not believed that severe turbulence could have occurred unless the flight entered a rotor directly downwind from (north of) the island’s 7000 ft peaks.
  • Altimetry problems. The pressure was highly typical for tropical locations (1017 hPa, or 30.03 in Hg). If the aircrew had neglected to change their altimeters from 1013 to 1017 hPa from top of descent then the altimeters would have read about 120 ft low, thus controlled flight into the ground based on altimeter omissions can probably be ruled out; furthermore the warm tropical atmosphere causes the flight levels to be slightly higher than indicated.

    References
    * Agee, E.M., Chen, T.S., and Doswell, K.E., 1973. A Review of Mesoscale Cellular Convection, Bull. Amer. Met. Soc., Volume 54, Issue 10 (October 1973) pp. 1004–1012.
    * Siquerra, J.R., Rossow, W.B., Machado, L., and Pearl. C.. (2005). Structural Characteristics of Convective Systems over South America Related to Cold-Frontal Incursions, Mon. Wea. Rev., Volume 133, Issue 5 (May 2005) pp. 1045–1064.

    Thanks to Tim Vasquez

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