Aircraft encounters with weather balloons: risks and mitigations - PowerPoint PPT Presentation

Aircraft encounters with weather balloons: risks and mitigations Bob Lunnon Royal Meteorological Society

Background • There have been a number of incidents stemming from aircraft encounters with balloons, where the pitot systems on the aircraft have been affected. • As far as is known, none of these encounters have been with radiosonde balloons, and it is not clear, given that a radiosonde balloon is designed to burst, that such a balloon poses a threat to the pitot system and other measurement systems on aircraft. • This study considers the threat from radiosonde balloons and mitigations: one of the mitigations applies to all balloons and other causes of problems with pitot systems.

*Lerwick Reference stations (00z and 12z) * South Uist Automatic stations (00z only) MoD stations Albermarle * Castor (on demand) * Bay Watnall * * Aberporth * Valencia Larkhill * Herstmonceux * * Camborne

Current use of radiosondes in the British Isles and elsewhere • Radiosonde stations in the British Isles fall into 3 categories. – Reference stations (Camborne, Lerwick, Valentia) release radiosondes twice daily, at 2315 GMT and 1115 GMT. – Automatic stations (Herstmonceux, Watnall, Albermarle and Castor Bay) release radiosondes daily, at 2315 GMT. – MOD stations (Larkhill, Aberporth and South Uist) release radiosondes as/when needed to support trials (e.g. artillery at Larkhill). Thus there are no regular releases of radiosondes along the SSE/NNW axis of Britain except at night when domestic passenger flights are minimal.

Current use of radiosondes 2 • Information on the web, for example, http://badc.nerc.ac.uk/data/radiosglobe/europe.html (which as advertised in an FSB article on In Flight Impacts) imply that there are 30 launch sites in the UK. • In fact there are 9 as described above. • That web link states that there are 200 sites across Europe: this figure is almost certainly too high.

Current use of radiosondes 3 • The nominal ascent rate of radiosonde balloons is 1000 feet/minute (between 5 and 5.5 m/s). This figure can be used to quantify the risk of an encounter at a particular time at a particular flight level.

Current use of radiosondes 4 • Use of radiosondes in other parts of the world follows a similar pattern to that in the British Isles. • Radiosondes are rather expensive and it is much more cost effective to obtain wind, temperature and is possible humidity information from commercial transport aircraft. • Therefore the use of radiosondes in areas where there is dense commercial air traffic will tend to be avoided at the times of day when air traffic density is at its highest.

Mitigation 1 – prediction of position of radiosondes • Radiosondes are released from well defined points at predictable times. • Assuming they are filled with a pre-set quantity of helium, their ascent rate is predictable. • Therefore the trajectory (in 4 dimensions) of the radiosonde is largely predictable – it depends on the wind at levels from the surface to the level of interest. • In principle an airline with access to forecast winds generated by the Met Office could predict the trajectory of any radiosonde anywhere in the world.

Prediction of position of radiosondes 2 • The involvement of Air Traffic Management service providers in the provision of predictions of radiosonde predictions is recommended. • One possible scenario is that individual Met Services who release radiosondes provide predictions of their positions to ATM providers controlling the airspace through which the radiosondes are expected to pass (this would take into account the three-dimensional structure of airspace). • The ATM providers would then vector aircraft round any radiosondes in their airspace.

Mitigation 2 – diagnosis of position of radiosondes • Radiosondes routinely broadcast their position (along with other met data such as temperature) and do so in one of only two frequencies – 403Mhz or 1680MHz. • There is nothing in principle to prevent a suitably equipped aircraft “listening in” to the transmissions of any radiosondes within radio range. • The position information could then be fed into a system such as TCAS which could then provide advisories (and other warnings) to the pilot recommending changes of flight path which would enable the aircraft to avoid the radiosonde.

Mitigation 3 – reduced reliance on pitot tube information • Radiosonde balloons, and other similar objects, pose a threat because of the risk of affecting the determination of airspeed using pitot systems on aircraft. • Other threats to measurements by Pitot systems A number of mechanisms can affect the performance of Pitot systems. These include (a) Icing, as affected flight AF447 (b) Volcanic ash (c) Bird strikes (d) Foreign objects (e) Balloons and other airborne objects made of rubber, e.g. banners • If a mitigation can be developed which works through reducing dependency on pitot systems, then this can be applied to the other causes of pitot unreliability.

Accuracy of components of the “wind triangle” • In the absence of any of the effects (a) to (e) above, airspeed has a typical RMS error of better than 1m/s. • The accuracy of the ground velocity vector is also very good, using a combination of Inertial Reference Systems (IRS) and Satellite Navigation Systems (typically GPS) giving a typical RMS error in either of the components of the vector of better than 1m/s. • Aircraft heading is a significant source of error in determining the wind vector as derived from airspeed and ground velocity, and wind has a typical RMS error in either of the components of ~1.5m/s.

Wind Triangle Wind vector Air velocity Ground velocity

Accuracy of upper level wind forecasts • Upper level winds are the most accurate forecasts the Met Office produces (compared to natural variability) and RMS errors have approximately halved in the last 20 years. • Statistics on accuracy are available on the Met Office website. • Currently 24 hour forecast winds at FL390 for the zone north of 20 o N have an RMS vector error of 3m/s. • This figure applies to average wind over 10-20km: for shorter distances there will be larger errors. • Shorter range forecasts have smaller errors. • The 3 m/s figure is for vector error: for a single component the RMS error will be 3/√2 which is approximately 2m/s.

Accuracy of upper level wind forecasts 2 • It is clear that in the absence of any of the effects (a) to (e) above, airspeed is more accurately determined from the pitot system. • However, in the presence or suspicion of any of the effects (a) to (e) above, use of forecast wind data coupled with ground velocity information from on-board sources can significantly reduce uncertainty. • For example, if the two pitot systems give different figures for airspeed, in many cases it should be possible to decide which of the two systems is more accurate using forecast wind information. • This was a noted aspect of flight AF447. – For the period between 2:10:04 and 2:10:26 the two computed airspeeds were significantly different 40% of the time; – for the period between 2:10:26 and 2:10:50 the two computed airspeeds were significantly different 70% of the time; – for the period between 2:10:50 and 2:11:46 the two computed airspeeds were significantly different 30% of the time. (See figures 26 to 28 of the BEA final report).

Indicated airspeed and true airspeed • In most contexts the critical quantity that a pilot will refer to is indicated airspeed rather than true airspeed. • In order to convert between the two it is necessary to make reference to outside air temperature and barometric pressure. • Although it does not follow that if the pitot system was not performing nominally anomalous measurements would be made by the air temperature sensor and/or the static pressure sensor, it is certainly true that air temperature sensors are prone to icing problems and foreign objects could affect any sensor. • Upper level forecast temperatures have a RMS error of 0.7 degrees which would give rise to a true airspeed error of less than 1m/s.

Indicated airspeed and true airspeed 2 • The forecast true heights of flight levels are also broadcast as part of the services provided by World Area Forecast Centres. • It is possible to combine the forecast heights with the geometric aircraft height derived either from the IRS or GPS to derive the flight level of the aircraft without reference to the static pressure. • It follows that if all relevant forecast information was available on the flight deck, an aircraft could fly without pitot systems, outside air temperature sensors or static pressure sensors.