Method and apparatus for determining the risk of aquaplaning

The present invention relates to the production of meteorological information for aviation and particularly for warning about aquaplaning risks at airports. More specifically, the invention relates to a method, according to the preamble of Claim 1, for determining an aquaplaning risk and an apparatus according to the preamble of Claim 8 for determining an aquaplaning risk.

Aquaplaning causes a surprising loss of adhesion, when a water layer accumulates between a tyre and the road, usually at high speed. Experiencing aquaplaning is a common problem for all vehicles equipped with rubber tyres, including aircraft. In fact, aquaplaning was a partial cause of an accident that occurred on 23 September 1999 in Bangkok, which led to 38 passengers being injured and to considerable material damage. General, aquaplaning that occurs in traffic is dynamic, so that for it to occur the vehicle must have a specific minimum speed and the water film between the road and the tyre must be of a specific minimum thickness. For example, the minimum speeds (in knots) of an aircraft beginning to aquaplane is typically 7 times the square root of the tyre pressure (in psi). As the tyre pressures of aircraft vary in the range 60 - 200 psi, the critical speed varies in the range 50 - 100 knots. The minimum thickness of the water film, for its part, depends strongly on the surface of the road and the tyre tread pattern, but generally it can be estimated as being about 2.5 - 5 millimetres. Because it is not possible to change the speed of, for example, an aircraft that is landing, sufficiently to eliminate the aquaplaning risk, the pilot must be warned of a rainwater accumulation of a dangerous depth, in which case landing can be made to take place as close to the start of the runway as possible.

As is known, the thickness of the water layer can be measured directly using sensors placed on the surface of the road, but considerable drawbacks are associated with this method. Surface-sensor arrangements are not especially reliable, because the sensors do not provide an accurate picture of the thickness of the water layer, nor does the method take into account, for example, the accumulation of water in depressions. At the same time, the installation of the sensors is expensive and a considerable number of them are required to provide adequate coverage, which increases not only costs, but also error sensitivity. In addition, direct measurement is made more difficult by dirt accumulating on the surface being measured, which changes the properties of the water, due to which it is difficult to obtain measurement data reliably and independently of surface dirt, using present surface sensors.

Procedures are also known, in which a remote-measuring device sensing road-surface or runway conditions, which the aid of which information on water accumulation and distribution on the runway is obtained, is attached to the vehicle. However, a method like that described is disadvantageous, because the areas being measured must then be kept clear for the measurement and there is no benefit from the measurement for other traffic. In addition, it requires resources and cannot be automated.

The invention is intended to solve at least some of the aforementioned problems and create a real-time indirect method for detecting an aquaplaning risk and warning of it.

In the method according to the invention, first the runoff properties of the area are charted by measuring the thickness of the water layer caused by rain on the area. After this, the measured water-layer thickness is compared with the estimated thickness, so that it is possible to define the area's computational runoff- time constant, which can be used when estimating the greatest possible thickness of the area's water layer. During estimation, the amount of rain on the area is measured and an aquaplaning warning is issued based on the computed value, if the time-period-specific thickness of the water layer exceeds a critical value. More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The device according to the invention comprises at least one rain gauge, which is arranged to transmit the measurement data, and a computing appliance, which is arranged to receive the transmitted data and which has means for placing the information received into a calculation equation. The computing appliance has further means for comparing the result of the equation with a predefined value, and means for issuing an aquaplaning warning, if the result of the equation is equal to, or greater than the predefined value. More specifically, the device according to the invention is characterized by what is stated in the characterizing portion of Claim 8.

Considerable advantages are gained with the aid of the invention. The computational estimation of the thickness of a rainwater layer accumulated on an area, on the basis of rainwater-accumulator measurement, is extremely cost-effective, as rainwater accumulations are measured, for example, in air traffic for other purposes too. At the same time, the simple method in question, which can be automated, is accurate and reliable. On the other hand, the area being monitored need not be closed to traffic except for the duration of the first stage, and, in the best case, this can be taken care of during regular maintenance breaks, which has further beneficial effects on costs. Above all, the use of the method provides important information on the surface of the road or runway, which information may not necessarily have be acquired before, for the aforementioned reasons. Simple estimation of the aquaplaning risk has a considerable beneficial effect on traffic safety.

In the following, embodiments of the invention are examined in greater detail with reference to the accompanying drawing.

Figure 1 shows a device for determining an aquaplaning risk according to one embodiment.

According to one embodiment of the present invention, the determining of an aquaplaning risk comprises two basic stages. In the first stage, the runoff properties of the area 1 are charted and, in the second stage, the rainwater accumulation in the selected area 1 is monitored, based on which the aquaplaning risk prevailing in the area is estimated.

The term runoff properties refers to the ability of the area 1 to be monitored to transfer accumulated moisture away from the surface of the area 1. For example, in aviation applications, it is natural to monitor a runway's ability to transfer the water accumulated on it to drains or other collection points, for example, to a neighbouring green area 4. Thus, the charting in the first stage of the runoff properties is of primary importance when estimating the aquaplaning risk. If the area to be estimated is extensive and comprises more than one runway, the runway area can be divided into several sub-areas, each of which is charted separately. Thus, the runway aquaplaning warning for precisely the right runway can be transmitted to an aircraft landing on a specific runway.

Thus, in the first stage, a set of points A - E is selected from, for example, an airport runway 1, the thickness of the rainwater accumulating on which is measured during a rainy period. The more points there are, the more comprehensive the sample obtained, and the greater the probability that the critical points on the airport will be found, for example, depressions, in which rainwater easily accumulates. Preferably, it would be good to location the monitoring points A - E at least at both ends of the runway. The thickness of the accumulated rainwater can be measured in many ways, but measurement is preferably performed by optical remote measurement. Arrangements are previously known, in which a remote-measuring device, which senses surface conditions, is attached to a vehicle, with the aid of which it is possible to obtain a comprehensive sample of the water accumulation on the runway, and its distribution. Particularly in runway applications, the installation of corresponding devices on top of observation posts is challenging, as it is not possible to erect additional high structures in an airport area.

What is important, however, is that the thicknesses of the water layers accumulated on the surface of the area of the runoff properties charted in the first stage are measured. It is preferable to measure the thicknesses of the water layer at several different thicknesses, for example, five measurements in the range 1 - 5 mm. At the same time, the overall rain accumulation of the area is measured using suitable measuring devices 2. Once the amounts r(i) of water that have rained on the areas being monitored over a specific period of time i are known, and the thicknesses (t(i) of the rainwater accumulations measured at corresponding intervals of time are known, the runoff-time constant k of the area being monitored can be defined with the aid of equation 2 shown later. Depending on the area, the runoff- time constant k can be a constant, or it can depend on the thickness t of the rainwater accumulation, for example:

or some other functional dependence of the thickness of the water surface. It is easy to add other dependencies to the equation, for example, the effect of a crosswind of the flow properties. The first stage of determining the risk should be repeated at regular intervals, and always when structural changes have been made to the area being examined.

Once the runoff properties, i.e. the runoff-time constant k of the area 1 have been determined, on the basis of this the aquaplaning risk can be estimated computationally, without new local surface charting. By means of computational estimation, reliable accuracies are achieved while the runway need not be kept clear of air traffic for the duration of the measurement. In the second stage of determining the aquaplaning risk, the rainwater accumulation of the area is thus measured in the same way as in the first- stage charting of the runoff properties. By utilizing the rainwater accumulation r(i) measured over a specific period of time and the runoff constant k, the thickness t(i) of the time-period-specific water layer can be estimated computationally, using the equation:

in which

-t(i) is the time-period-specific water-layer thickness in millimetres, -t(i - 1) is the water-layer thickness of the preceding period of time in millimetres,

-k is the runoff-time constant, and -r(i) is the time-period-specific rainwater accumulation in millimetres. The time period is preferably one minute, so that the rainwater accumulation is measured and the thickness of the water layer is estimated at one-minute intervals. Thus, using equation (2), the runoff-time constant k is also determined, once several implemented water- layer thicknesses t(i) and rainwater accumulations (r(i) are known.

The estimated computational time-period-specific water-layer thickness t(i) can thus be compared with the critical limit of the water layer, a thicker water layer of which can be regarded as causing an aquaplaning risk for a landing aircraft. Typically, such a critical limit is about 2.5 - 5 millimetres. If the estimated thickness (t(i) is greater than, or equal to the defined critical limit, air traffic is informed of the aquaplaning risk.

The apparatus according to the invention is typically implemented according to Figure 1, in such a way that several rain gauges 2 are placed on the area 4 of the airport. The amounts of rain measured by the rain gauges 2 are used in the computing appliance 3 together with the runoff properties of the airport area 4 to create an aquaplaning warning.

The rain gauges 2 can be so-called collector-type rain gauges, using which the rain is collected in a vessel and the vessel is weighed, or measuring devices based on corresponding hydrometeorological detection, wherein the type of the hydrometeor and the amount of water is detected on the basis of the amount of hydrometeorological motion and the signal it forms, while the total amount of rain is estimated on the basis of the sum of the hydrometeorological impacts detected.

The computing appliance 3 can be any computer whatever with sufficient computing power, equipped with suitable software.

Telecommunications can be implemented either wirelessly, or in a wired form from the rain gauges 2 to the computing appliance 3 and if necessary, forwarded to field personnel, for example, wirelessly to palm computers.