# Why are flight recorders generally located in the tail?

Is there a particular reason why FDRs and CVRs and associated similar bits of equipment are kept in the tail section of a plane?

When an aircraft is crashed, the entire front of the aircraft is expected to act as a crush zone. By placing FDRs and CVRs near the tail, the shock wave that reaches FDRs and CVRs is reduced significantly.

The probability of survival based on seating is shown in this picture:

Hence one can see that the shock wave reduces as it reaches the rear of the plane. That is the reason FDRs and CVRs are installed in aircraft's empennage section.

Please note that the precise location of the recorders depends on the individual plane. Sometimes they are located in the ceiling of the galley, in the aft cargo hold or in the tail cone that covers the rear of the aircraft.

• And Business Class passengers actually pay for the privilege of travelling inside the crush zone. Jan 13 '15 at 1:40
• @PieterGeerkens Shhhh! Don't tell them that.
– Farhan
Jan 13 '15 at 1:41
• @PieterGeerkens and this is actually a great incentive for pilots to not crash. Jan 13 '15 at 1:48
• @MarchHo depends on how those percentages are sold/read: in the current state they mean "if there are survivors at all, this is where is likely to find them", but most people expect a chart saying "if there is a crash, this are your chances of survival", something you obtain including crashes with no survivors.
– Federico
Jan 13 '15 at 8:08
• @Federico And it's very important to include the crashes where everybody died. The difference of 69% at the back versus 49% at the front looks huge but it might be an artifact of the data selection. As you add more and more crashes with 100% fatalities to the data, those numbers will get closer and closer together (and closer to zero). Jan 13 '15 at 10:29

The reason for the FDR and CVR to be located in the rear of the aircraft is because that gives the best chances of recovering them.

Usually an aircraft has a forward speed when it crashes. The initial shock of impact is absorbed by the forward part of the aircraft, resulting in a more gradual deceleration for the aft parts as the aircraft breaks up. By putting the FDR and CVR in the back the acceleration is reduced allowing for a higher probability of them remaining intact.

• The tail also tends to crash 'slower' if the plane breaks up, due to the drag of the stabilisers. A minor point with regard to the major one that 'it's the place they're most likely to survive' Jan 13 '15 at 1:33

In addition to Farhan's excellent answer, it should be noted that reducing the peak impact shock is not the only reason that the FDR is placed in the tail, and that the passenger survivability is higher behind the wings.

The wings are full of fuel, and when the tanks rupture during a crash the fuel tends to spill forward. If you look at crash videos you will notice two common scenarios:

1. In incidents in which the plane scrapes along the runway, friction with the ground starts a fire. In this scenario the rear of the plane is engulfed in flames, but that is the exterior. When the plane comes to rest, either the whole vehicle is engulfed or the fire recedes to small flames on the parts touching the ground: the engines and undercarriage.

2. In incidents in which the plane collides with the ground and does not scrape, the fuselage and wings rupture and fuel spills forward. In these cases the fires are mostly in the front portion of the craft, and the broken fuselage allows the fire to quickly spread to the interior of the craft. These fires are much more dramatic and look like explosions due to the high quantity of fuel and fuel vapor being available.

In the USA, there is a legal requirement

(e) The recorder container must be located and mounted to minimize the probability of rupture of the container as a result of crash impact and consequent heat damage to the recorder from fire.

(1) Except as provided in paragraph (e)(2) of this section, the recorder container must be located as far aft as practicable, but need not be outside of the pressurized compartment, and may not be located where aft-mounted engines may crush the container during impact.

(2) If two separate combination digital flight data recorder and cockpit voice recorder units are installed instead of one cockpit voice recorder and one digital flight data recorder, the combination unit that is installed to comply with the cockpit voice recorder requirements may be located near the cockpit.