View Full Version : What is the Water-line?

5th Jul 2006, 18:00
Hi Ladies and Gents

I was reading an aviation article and came across the aircraft Water-line. For the life of me I cannot remember how it is defined for an aircraft.

Does anyone wish to enlighten me?



Mad (Flt) Scientist
5th Jul 2006, 18:06
It's usually an arbitrary drawing reference: the fuselage will be referenced to "fuselage station" (distance aft of some arbitrary plane forward of the nose), "water line" (distance above (usually) or below some arbitrary horizontal plane, (usually) below the aircraft, and "butt line", distance left or right of the plane of symmetry.

There's no actual physical water involved.

5th Jul 2006, 20:06
The waterline is also used as a reference for pitch attitude on the ground, which sometimes enters into calculations for weight & balance (especially in automated systems), proper strut inflation, manual dipping of fuel tanks, etc.

Don Coyote
5th Jul 2006, 21:14
I thought the water line was with regard to ditching. All hatches, like outflow valve etc which are below the water line should be closed before ditching.

Could be wrong though!

5th Jul 2006, 21:58

Taking in mind that aircraft engineering nomenclature uses some of the more naval expressions such as "port side", "starboard side", "water line" etc this might have been the explanation.

However as my fellow colleague (MadFlightScientist) said this is only a reference meaurement giving the distance from a reference horizontal plane. Usually water line 0 is set at the lowest point on the fuselage where it is the thickest.

For example this could be the lowest postioned stringer on the bottom of the fuselage, if there is any.

In general all water line coordinates are positive.


6th Jul 2006, 06:41
Aircraft design followed naval architecture standards of the day during the birth of aircraft design, and much stays today. Hence water-lines, hull stations, etc.

Waterlines are horizontal sections through a hull and are arbitrarily assigned by the designer, having no real reference even in boat design to the actual waterline, apart from sharing the plane. In boat design, WL0 tends to be the actual "load water-line" or LWL.

9th Jul 2006, 16:07
Don Coyote is correct. JAR 25.807 defines the certification parameters for aircraft emergency exits. This document uses the term "waterline" in the way Don Coyote described. Below is an extract to illustrate this:
"(2) For aeroplanes that have a passenger seating configuration of 10 seats or more, excluding pilots seats, one exit above the waterline in a side of the aeroplane, meeting at least the dimensions of a Type III exit for each unit . . ." etc.
The above quote is taken from the paragraphs that refer to emergency egress and ditching. There are other similar references to the "waterline" in JAR 25. As Dan suggests, some aircraft have a "Ditching" pushbutton which has to be pressed as part of the ditching drills. It closes all doors and vents below the waterline to minimise water ingress following a ditching. Examples are the avionics inlet and outlet doors, the pressurisation outflow, ac pack outflows etc.
The other contributers may also be correct in their references to the nautical terms used historically in aircraft design, but I only claim any knowlege of the JAR 25 use of the term.