Muhammad Kamal Aero-10 110101021

The DeHavilland Comet Crash

Case Study

The DeHavilland Comet was the first production commercial jet airliner that

went into service in 1952. The earliest production aircraft designated G-ALYP

was loaned to the British Overseas Airways Company and inaugurated the first

scheduled overseas flight from London to Johannesburg with fare-paying

customers on-board. Much of the design is similar to the commercial airliners

seen around the world today. The Comet had four turbojet engines (turbofan

are now the norm for reduced noise and better fuel economy), which made the

aircraft much more efficient at higher altitudes of flight than its propeller-

driven contemporaries. Furthermore, it featured an internally pressurised

fuselage/cabin and also pioneered design elements which were unusual at the

time such as backward-swept wings, integral wing fuel tanks and a four-wheel

bogie undercarriage (1). Unfortunately, the DeHavilland Comet also influenced

modern aircraft design by two catastrophic failures.

Within two years of entering service two of the Comet fleet fell apart during

ascent to cruise altitude with a total loss of the aircrafts and the death of 56

passengers. The first production aircraft G-ALYP, scheduled on BOAC Flight

781 from Rome Ciampino to London Heathrow, was lost on January 10, 1954 by

the fuselage breaking up in mid-air 20 minutes after taking off. BOAC

voluntarily grounded its fleet and engineers suggested 60 immediate

modifications to the design to rectify some of the design flaws that were

believed to have caused the accident (2). Comet flights resumed on March 23,

1954 but only two weeks later on April 8, 1954 Comet G-ALYY, on the

chartered South African Airways Flight 201 from Rome Ciampino to Cairo,

again crashed into the Mediterranean sea within 30 minutes of take-off. The

entire Comet 1 fleet was then grounded, its Certificate of Airworthiness

revoked and the line production at DeHavilland in Hatfield suspended.

A number of investigations followed led by Sir Arnold Hall at the Royal

Aeronautical Establishment in Farnborough, UK. Most critically this included a

full-scale cyclic internal pressurisation test of the fuselage in a water tank of

the aircraft G-ALYU removed from service for this purpose. G-ALYU had

accumulated 1221 internal pressurisation cycles in service and after a further

1836 cycles in the water tank the cabin ripped open after a proof-test loading

33% higher than the nominal pressurisation cycle loading (2). Evidence of

fatigue cracking was found that originated from the aft lower corner of the

forward escape hatch and also from the right-hand aft corner of the windows

illustrated in Figures 1 and 2 below.

Fig. 1. Failure origin in Comet G-ALYU around escape hatch (1, 2).

Fig. 2. Failure origin of Comet G-ALYU around square windows (1, 2).

Both of these locations feature sharp right hand corners which cause local

areas of high stress-concentration that provide very benign conditions for crack

initiation and propagation under fatigue loading. Furthermore, circular

cylindrical structures, such as the aircraft fuselage, develop internal membrane

stresses (constant through the thickness) to resist the internal pressure loads.

As a result of the curved shape of the fuselage these forces induce secondary

out-of-plane bending moments acting to “straighten-out” the curvature. In

addition, the stress concentration around the the escape hatch and window

cutouts was exacerbated by countersunk bolt holes creating a “knife-edge” in

both the primary skin and doubler reinforcement (Figure 3) (2). Swift (1987)

has argued that the shell structure would have had enough residual strength to

sustain large and easily detectable cracks if they had grown midway between

two window cutouts. However, cracks that grew across a bay from one cutout

to the next would not be tolerable and result in ultimate failure of the

structure.

Fig. 3. Failure origin of Comet G-ALYU around countersunk bolt holes (1, 2).

Lessons Learned

The most notable lesson learned from the Comet disaster is that viewing

windows are no longer designed square but with rounded edges to reduce any

stress concentrations. Another immediate lessons is that crack-stoppers are

now placed between frame-cutouts that take the shape of circumferential

stiffeners that break-up the fuselage into multiple sections and thus prevent the

crack from propagating from one window to the next. Most importantly

however, before and during the Comet era the aircraft design philosophy was

predominantly SAFE-LIFE, which means that the structure was designed to

sustain the required fatigue life with no initial damage and no accumulation of

damage during service e.g. cracking (1). The Comet accidents showed that

around stress concentration cracks would initiate and propagate much earlier

than expected, such that safety could not be universally guaranteed in the

SAFE-LIFE approach without uneconomically short aircraft service lives.

For this reason the FAIL-SAFE design philosophy was developed in the late

1950’s. All materials are assumed to contain a finite initial defect size before

entering service that may grow due to fatigue loading in-service. The aircraft

structure is thus designed to sustain structural damage without compromising

safety up to a critical damage size that can be easily detected by visual

inspection between flights. All inspections are coupled with crack propagation

calculations that guarantee that an observed crack is not susceptible to grow to

the critical size between two inspection cycles, in which case adequate repair is

performed. Furthermore, the structure is designed to be damage tolerant with

multiple load paths and built-in redundancies that impart residual strength to

the aircraft in case the primary structure is compromised in-service.

References

(1) R.J.H Wanhill (2002). Milestone Case Histories in Aircraft Structural

Design. National Aerospace Laboratory. NLR-TP-2002-521

(2) T. Swift (1987). Damage tolerance in pressurised fuselages. 11th Plantema

Memorial Lecture. New Materials and Fatigue Resistant Aircraft Design (ed. D

L Simpson) pp 1 – 7. Engineering Materials Advisory Services Ltd., Warley, UK.