Characterization of dynamic hydroplaning speed for large transport aircraft equipped with a multi-row main landing gear
Creators: name: LAGUNAS, Sara
Publisher: name: Proceedings of the 8th European Conference for Aeronautics and Space Sciences. Madrid, Spain, 1-4 july 2019
Year: 2019
Subjects: FLIGHT TESTS
AEROSPACE VEHICLE DESIGN
AIRCRAFT
FLUID MECHANICS

description: In the context of ground operations on water contaminated runways, hydroplaning constitutes a critical condition, which leads to strong wheel spin-down and substantial loss of braking friction, and may result in undesired runway excursions. Therefore, an adequate knowledge of the ground speed at which dynamic hydroplaning starts to develop (hydroplaning speed, Vp) becomes essential. EASA (AMC 25.1591) and FAA (AC 25-31) accept the characterization of Vp as a sole function of tire inflation pressure (p), by means of formula Vp = 9p (Vp in kt, p in psi). Such relation was established in 1963, based on the results of ten unique test points. This paper attempts to bring into question the relevance of this relation. Additionally, neither EASA nor the FAA allow for the consideration of a variation in the hydroplaning definition above in multi-row undercarriages, in which not all wheels are equally affected by the contaminant: front wheels both displace fluid from the path of aft wheels and impinge fluid on them. Therefore, a non-negligible variation in Vp might be found. Another aspect assumed by the aforementioned advisory material is that, in terms of braking performance, hydroplaning conditions are analogous to icy ones, accepting a characteristic braking friction value of 0.05. The present study pursues two different objectives. The first is to assess the differences in the onset of hydroplaning depending on wheel position (front or aft). The second is to evaluate if the advisory braking friction proposed by EASA and the FAA to be applied in hydroplaning conditions, is adequate for multi-row MLG assemblies. A series of taxi tests were conducted on a specifically built pond facility, simulating highly contaminated conditions (15 mm water on average) in support of the study. The selected test aircraft was equipped with a dual-tandem MLG. All tests were performed on the same runway, of sufficient macro texture to provide adequate drainage. All tires had equal type, dimensions, and inflation pressures, and were circumferentially grooved. The ground speed (Vg) range covered was 60-110 kt. The criterion established for the detection of Vp was a combination of wheel spin-down and residual to null braking torque. Both parameters were monitored at 128 Hz. Hydroplaning was apparent on front wheels from 100 kt. Nevertheless, no spin-down was observed on aft wheels, and their braking torque was not as highly degraded. Results suggest that the MLG configuration plays a part in the onset of hydroplaning at aircraft level, and it therefore may be considered as an additional parameter for the characterization of Vp. At front wheel level, the observed correlation between tire inflation pressure and Vp was Vp = 8.4p. In contrast, aft wheels did not reach hydroplaning over the entire ground speed range tested. Further hydroplaning-dedicated testing is recommended, with a view to provide further evidence to support a review of the current certification standards.