Measuring and testing – Tire – tread or roadway – Tire inflation testing installation
Reexamination Certificate
2002-10-15
2004-08-24
Oen, William (Department: 2855)
Measuring and testing
Tire, tread or roadway
Tire inflation testing installation
Reexamination Certificate
active
06779392
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a tire testing device, and in particular to a device for testing the tires of aircraft.
1. BACKGROUND TO THE INVENTION
Aircraft tires are required to contain a maximum of 5% oxygen. The reason for this is that if tires become heated to more than approximately 200° C. (which may occur if a set of brakes is binding or dragging), the chlorobutyl in the tire material begins to break down and to produce isoprene, and isoprene and oxygen together form an auto-ignitable mixture. The auto-ignition of aircraft tires might have been the cause of an unknown number of previously unexplained losses of aircraft.
The instance of such auto-ignition is likely to increase because it is becoming increasingly common to fit aircraft with carbon brakes. Carbon brakes are advantageous since they are lighter, more efficient and longer-lasting than conventional brakes. However, whilst conventional brakes melt at around 400° C., carbon brakes are effective up to around 1100° C., so that the temperature which can be generated in a binding brake, and therefore which can be transmitted to an aircraft tire, is correspondingly increased.
Because of the known problems with auto-ignition, there is a mandated limit of 5% oxygen in aircraft tires; the presence of such low concentrations of oxygen prevents auto-ignition in the presence of isoprene.
Aircraft owners and users seek to meet this limit by filling the tires with nitrogen. Aircraft tires may require a pressure of around thirty atmospheres, for example, and so the air which is present in the tire before inflation becomes diluted by around thirty times. The air which was present in the tire before inflation will typically be atmospheric air containing around 21% oxygen; diluting this thirty times with pure nitrogen will result in an oxygen content within the tire of 0.7%, well within the mandated limit.
However, the nitrogen which is used to inflate (or reflate) the tire will seldom if ever be pure, and in certain cases might contain several percent oxygen. It is necessary that the nitrogen supply contain less than 4.3% oxygen (for a thirty atmosphere pressure tire) so that the 5% level can be met.
In many cases, the owners of aircraft will use liquid nitrogen to inflate and reflate the tires of their aircraft, and this supply can be close to 100% pure in practice. However, liquid nitrogen is expensive and other less diligent owners and users instead utilise pressurised nitrogen gas. Often the pressurised nitrogen gas is purchased primarily by price, and the quality (i.e. the percentage of oxygen present in the gas) is not certified and may not be known.
Also, at some airports the ground staff are not qualified or are not trained to appreciate the significance of the oxygen content of the tires, and it has been known that tires be filled from an oxygen supply when the nitrogen supply was not available!
In addition to the oxygen content of the gas within the aircraft tire being critical, the pressure of the gas within the tire is also important. Thus, a tire which is under-inflated does not offer the same level of grip as it is intended to provide, and under-inflated tires are believed to be particularly suspectible to aquaplaning or hydroplaning when there is standing water on the runway (where a layer of water becomes trapped between the tire and the surface of the runway, reducing the level of grip therebetween). There have been a number of aircraft accidents in which the aircraft has skidded off the runway, and aquaplaning is believed to be a likely cause of the aircraft's failure to stop.
2. DESCRIPTION OF THE PRIOR ART
Pressure testing devices for the tires of aircraft are well known. One form comprises a mechanical gauge similar to that first invented over one hundred years ago. More modern devices use an electromechanical sensor.
However, with such devices it is only feasible to test the pressure of the tires when they are at a known reference temperature, and this typically means that the tire must be at or close to the ambient temperature, otherwise the hot gas within the tire will be at a greater pressure than the corresponding gas when cold, and the temperature-induced variation will render the pressure reading unreliable.
In the United States, for example, the Federal Aviation Authority (F.A.A.) has expressed the wish that tire pressures be tested every day, but the airlines have indicated that this cannot be achieved in practice because aircraft are often in continual operation for up to three weeks at a time, and the aircraft is not on the ground within this period for long enough for the tires to cool sufficiently for reliable testing to take place.
Oxygen testing devices are also available, by which the oxygen content of an aircraft tire can be tested. However, the use of these devices is not universal because of the time taken to undertake the testing. Thus, it has been estimated that to test the pressure and oxygen content of every tire on a large aircraft can take up to two hours, and this is longer than the desired turn-around time for most aircraft (regardless of the time necessary for the tires to cool sufficiently for a reliable pressure test to be carried out).
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide a tire testing device which can be used when the aircraft tire is hot or cold.
It is another aim of the present invention to provide a tire testing device which can test the pressure and oxygen content of the tire in a single operation, i.e. only a single application of the valve head upon the tire valve needs to be undertaken.
According to the invention therefore, there is provided a tire testing device including a pressure sensor, characterised in that the device also includes a temperature sensor.
Preferably, the device has means to store a record of the volume of the tire, and means to calculate an effective pressure at a reference temperature. In the preferred embodiments the effective pressure at a reference temperature can be calculated by the device, but in other (less preferred) embodiments the pressure calculation can be carried out separately, e.g. by a computer or other device to which the measured pressure and temperature are downloaded.
It is known that the relationship between the pressure, temperature and volume of a given quantity of gas are related to each other (by Boyle's Law), and knowledge of the volume of gas within the tire can enable a pressure reading at any particular temperature to be converted to a pressure reading at another (reference) temperature. Accordingly, if it is determined that the pressure of the tires should be measured at 0° C., and the actual pressure is measured at 50° C., then the equivalent or effective pressure at 0° C. can readily be calculated.
The ability of the device to measure the tires at any given temperature, and in the preferred embodiments to calculate the pressure at a reference temperature, enables the device better to check any leakage of gas which is occuring from a tire. Thus, with conventional pressure test apparatus it would still not be possible to obtain any really useful data even if an aircraft's tires were allowed to cool and be measured every day, if the same aircraft is present in a hot atmosphere such as Arizona on one day, and a cold climate such as Alaska on the following day, since the ambient temperature, and thus the temperature of the “cold” tire, might differ by 40° C. between the two locations.
Preferably, the device comprises a base unit and a valve head, the valve head being connected to the base unit by a flexible tube, the valve head being adapted to connect to the tire valve and to allow the escape of a small amount of gas therefrom. Preferably also the temperature sensor is located within the valve head, so that the temperature of the gas is measured as close to the tire as possible, and the likelihood of miscalculations occuring, because for example the gas has cooled on leaving the tire, are much reduced.
Desirably, the temperature sensor is a t
Bell Colin Frederick
Knotts John Martin
Fuierer Marianne
Hultquist Steven J.
Newbow Aerospace Limited
Oen William
Yang Yongzhi
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