# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Aircraft position when you first hear it**

**From:**Antoine Couëtte

**Date:**2021 Oct 13, 05:38 -0700

Good day to you, David.

In reply to your *interesting remark*, I have often noticed that if and when I start hearing a subsonic civilian aircraft at its cruising altitude, it is almost approaching its closest point from me, i.e. it is almost already reaching its highest elevation above the horizon for me.

*I think that both our statements are compatible.*

*Let's verify whether they are actually.*

As an example :

An ** subsonic** A/C flies at 33,000' (FL 330) is heard by a sea level Observer. Average sound speed is 300 m/s over the

*slant*distance from A/C to Observer. A/C speed is M.78, in other words, the A/C flies horizontally at 234 m/s. We need 2 extra assumptions to fully define our problem.

The A/C ground track minimum distance relatively to the Observer needs to be specified for each case. And :

This second assumption is subject to change (e.g. Engines noise intensity and Observer's hearing abilities) but - at least for me - the following one seems reasonable: on the average I have noticed that I can only hear Aircraft flying at 20 Km or less from me.

Hence, let us compute the following 2 cases :

**A - The Aircraft overflies the Observer, i.e. A/C ground track minimum distance is equal to 0 km**

(A1) What is initial sound direction elevation when Observer first starts hearing it ?

(A2) What is A/C elevation when Observer first hears it ?

(A3) What is the Distance between A/C position when it first becomes audible to the Observer and its actual position then ?

**B - Same questions for an Aircraft with ground Track Distance minimum distance equal to 10 km.**

(B1) What is initial sound direction elevation when Observer first starts hearing it ?

(B2) What is A/C elevation when Observer first hears it ?

(B3) What is the Distance between A/C postion when it first becomes audible to the Observer and its actual position then ?

Any taker for this computation ? Of course I will publish results.

Antoine M. "Kermit" Couëtte

*antoine.m.couette[at]club-internet.fr*