Bjorn’s Corner: New aircraft technologies. Part 5. Optimal fuselage

Sep 23, 2023

As I noted above, the space under the floor used for baggage/cargo compartments is not wasted on the airlines that I fly on; rather it is used for baggage and cargo.

What about the space under the floor that is not used for baggage/cargo compartments? Is it wasted space? Anyone who has ever walked a ramp or maintenance hangar and looked into these spaces will know that they are not empty wasted space, but rather space filled with electronics racks, electrical cables, hydraulic lines, and ventilation and pressurization units and ducting. Do those suggesting otherwise imagine that all the instruments and displays on the control panel are run off of an iPhone or iPad glued to the back of the control panel, and connected to all of the equipment controlled from the cockpit by a single USB cable? Do they imagine that large air conditioning and processing units are not required to mix hot engine bleed air with outside air at (-60 C to -75 C at cruise altitudes) and send it at the right temperature and pressure to the cabin and to the wing for anti-icing, or that this is done without having to have processed air ducts run under the floor and out to the wings? Do they imagine that the equipment that converts electrical signals to commands to the hydraulics that move control surfaces, and the hydraulics powerful enough to move the control surfaces, do not take up substantial space? Do they imagine that the APU in the tail does not require a fuel line under the floor from the wing fuel tanks, or that there is not a need for electrical distribution panels where electrical power from the APU can be fed into the main electrical bus? Do they imagine that hundreds or thousands of sensors in the engines and control surfaces and their actuators do not need to have their data fed back to the flight deck through hundreds and thousands of cables through racks of analog to digital converters, digital to analog converters, signal amplifiers, signal conditioners, and network controllers? Do they imagine that all of this equipment, which must in most cases be redundant with duplicate line runs, processors, and computers, does not take up a significant amount of space or that it is not necessary to space the equipment out so that maintenance personnel can move (or at least crawl) around it when maintenance is needed?

All airliners that I am familiar with have a forward avionics/equipment bay under the cockpit and ahead of the forward baggage/cargo compartment, lots of equipment and interfaces in the wing box area to communicate with and control the engines and wing control surfaces, and an aft equipment bay behind the aft baggage/cargo area for controlling and communicating with the tail control surfaces and APU. The redundant air conditioning and pressurization packs are also usually near the wing box area. To get an idea of the size of the air conditioning packs, take a look sometime at the air conditioning units on the roof of a commercial building.

I have included below some links to videos of airliner underfloor equipment and avionics bays for the benefit of anyone who has never had a chance to look in these compartments, and is-having trouble imagining the size of the necessary equipment in the non-wasted space of these compartments.

787 Forward and Aft Avionics and Equipment Bays.

https://youtu.be/wDeQ88Z_9Pk

A350 forward avionics and equipment bay. Note that when the pilot enters the baggage/compartment from the equipment bay, there is not, even in this large wide body aircraft, enough room for an adult to stand up. The designers of this aircraft wisely and efficiently provided only enough height in the baggage/cargo compartment for the 64 inch (5 foot 4 inch) tall LD3 containers that this aircraft is designed to use. I would hope that no one is going to silly enough to suggest that baggage/cargo compartments on wide body long haul aircraft are wasted space.

https://youtu.be/LxS2RR-Vx_s

A320 forward avionics/equipment bay under the cockpit floor.

https://www.youtube.com/watch?v=rd960o1nK8M

A320 aft avionics/equipment bay.

https://www.facebook.com/a320systems/videos/a-look-inside-the-aft-right-avionics-bay-with-a-rack-of-flight-control-egpws-nav/734750576646647/

E190 forward avionics/equipment bay under the cockpit floor.https://www.youtube.com/watch?v=TmGl86l7ssc