Ivan de Lépinay, ENVISA, Paris, France
Laurent Cavadini, EUROCONTROL Experimental Centre, Brétigny, France

Extended Abstract

The engine thrust level of aircraft is a key parameter in the assessment of the environmental impact of civil aviation, both on a local and a global scale. Indeed, thrust is used as an input in most of the noise and emissions modelling tools, like the FAA’s Integrated Noise Model (INM) or the EEC’s Advanced Emission Model (AEM). While other input parameters to these models, such as the aircraft position, altitude and speed, can be easily obtained from radar data or flight simulations, information on the aircraft thrust level remain very difficult to find. Hence the necessity to calculate it using a “reverse-engineering” approach, on the basis of the available data (position, altitude and speed).

As far as noise assessment is concerned, the phase of approach is particularly complex, since the airframe noise component during this phase may exceed the contribution of engines noise itself. Therefore, efforts are being carried out to better evaluate the impact of the aircraft configuration (flaps and gear) on the overall noise on one hand, and on the engines thrust level on the other hand.

So far, the common way of assessing the aircraft thrust during the approach phase relies on a total zero force balance equation (static equilibrium), assuming the aircraft speed remains constant throughout the entire descent. It is also assumed that the drag over lift ratio does not vary with speed either, but only depends on the aircraft flaps and gear configuration. This method shows obvious limitations, especially in the situation when levels with deceleration phases due to airport-specific procedures are to be modelled.

The proposed paper presents the results of the first investigations that have been carried out on developing a more accurate thrust equation, based on a dynamic equilibrium including the aircraft acceleration or deceleration, and taking into account the variation of drag and lift coefficients with speed. First, the new equation is derived using basic theory of aerodynamics. Drag and lift coefficients are derived from the aircraft polar coefficients as available from the Base Of Aircraft Data (BADA). Then the equation is tested on a few specific aircraft types, for which manufacturer-provided approach procedures and profiles are available.

Results show a fairly good correlation between the calculated thrust and the one given by manufacturer, especially during idle thrust phases. Main differences appear during phases of quick configuration changes, mainly because of some imprecision in the assessment of drag and lift, and also because configuration changes are modelled as instantaneous. A constant shift in thrust is also observed during the final approach segment, which is probably linked to the aircraft angle of attack. These encouraging results should lead to further work in the assessment of the aircraft drag and lift as a function of its configuration, speed and angle of attack.