Failure Knowledge Database / 100 Selected Cases

Mid-air Explosion of Comet I over the Mediterranean Sea
Jan. 10th, 1954, Over the Mediterranean Sea (near Rome)
KOBAYASHI, Hideo (Yokohama National University)
TERADA, Hiroyuki (Japan Aerospace Technology Foundation)(Summary)
On Ja nuary 10th, 1954, the Com et G-ALYP, th e f irst jet air transport wit h a press urized cabin i n theworld, exploded in m id-air as a result of fatigue failure of the fuse lage structure at an alt itude of 8,000m
near El ba o ver the Me diterranean Sea, after taki ng-off fro m Ro me. All th irty-five p eople o n b oard w ere
killed in this accident.
1. Component
Fuselage
2. EventThe C omet was de veloped right after W orld War II a t de H aviland Aircraft Com pany i n U K, and it
started servi ce op eration i n May of 1952. T he first Comet, G -ALPY opera ted by BO AC, expl oded inmid-air at an altitude of 8,000m over the Mediterranean Sea near El ba after t he take-off fro m Champinoairport, Rome. All thirty-five on board (6 crew and 26 passengers) were killed in this accident.
Figure 1 shows the C omet 1 air transport. Figure 2 s hows the parts of th e aircraft that were retrieved
from the Mediterranean Sea by the salvage operation.
3. Course
The aircraft crashed after only 1,290 flight cycles (3,600 flight hours). This was only about one-tenth ofexpected design life. Following this accident, all flights of the C omet air transports were canceled, and theentire fleet was carefully inspected. 60 structural items that were thought to be suspect were reinforced. InMarch, t he British A viation Aut hority i ssued permission for resum ption of the operation, a nd BOAC
recommenced the flights.
However, a similar accident occurred again on April 8th, 1954 over the Mediterranean Sea near Naples
involving a n improved Comet air transp ort, G -ALYY. A ll tw enty-one p ersons onboard w ere ki lled th istime.
After a large-scale and exhaustive investigation, the direct cause of the accident was found to be fatigue
cracks emanated from the corners of the ADF window at the ceiling of the fuselage and the windows in thepassenger cabin that were caused by the pressurization cycles.
The fuse lage struct ure of a n aircraft h aving a pressurized c abin is s ubjected t o pr essurization f atigue
cycles caused by the pressure di fference between the cabin and the atmosphere o utside. Generally, cabinpressure is m aintained a t a round 0.8 atm , whil e the at mosphere pressure o utside the aircraft is ab out 0 .2
1

Failure Knowledge Database / 100 Selected Cases

atm when cr uising a t a n altitude of 12 ,000m in the stratos phere. Therefore , t he fuse lage s tructure is
subjected to a 0.6 atm pressure difference each flight.
In the fatigue design of the Comet, the effect of this pressurization cycle was taken into consideration tosome extent. During the development of the air transport, an internal pressure cycle test was conducted on
the fuselage structure.
Fatigue crac ks w ere first detected aft er 18,000 pressurization cycl es, and therefore the fat igue des ign
life was determined to be more than 10,000 flight cycles.
However, the accident occurred aft er far fewer flight cycles than this design life. The pressure patternapplied f or t he fat igue test using the a ctual fus elage str ucture is sh own in Fi g. 1. In t he fa tigue test , thepressure difference inside and outside of the cabin was assumed to be 0.56 atm. However, in the fatigue test
conducted on th e gr ound, a pro of press ure l oad c orresponding t o 1.12 a tm, that is tw ice that of ac tualpressure difference, was applied every 1,000 pressure cycles for safety purposes. As a result of this test, thefatigue crack initiation life was determined to be 18,000 pressure cycles.
The ef fect of proof press ure t est t hat w as n ot c onducted on actual aircraft for service was examined
during t he a ccident investigation stu dy. A fatigue test w as cond ucted o n an aircraft th at h ad e xperienced
1,230 serv ice fli ghts after applying a proof pressure t est of 0.75 at m. In t hat t est, fa tigue cracks w ere
detected after 1,830 pressure cycles. T he total of 30 60 cyc les is about one-sixth of the 18, 000 cycl esobtained by the fatigue tests with the higher proof pressure loads. This fact implied that the fatigue cycle
might approach 1,290 or 900 cycles of accident aircraft provided that proof pressure test was not conducted.
It was made clear that the proof pressure test conducted during fatigue test extended the fatigue life, and as
the result, proof pressure test during fatigue test could not give the safety side prediction of the fatigue life.
4. Cause
The following causal factors contributed to the occurrence of the accidents after a considerably smaller
number of flight cycles than the guaranteed fatigue life of 18,000 flight cycles.
(1) Following t he reg ulations of t he British Aviation Bu reau, a periodical proof p ressure t est with apressure load twice that of normal cabin pressure was conducted during the internal pressure fatigue
test. Crack initiation was observed aft er 18, 000 press urizing cycles in t hat test. However, t ensile
plastic def ormation w as induced a t t he corn ers of the w indows by th e pr oof pressure load. Th eplastically deformed zone form ed a compressive residual stress f ield when the proof-pressure wasremoved, w hich resu lted i n lon ger fat igue life, because a c ompressive resi dual st ress field has thesame effect as reduced mean stress conditions (See Fig. 2).
(2) Instead of the full-scale fuselage structure, only a part of the cylindrical fuselage fixed at the rigid steel
wall was provided for t he i nternal pressurization fati gue t est. T his tes t condition resu lted i n anunder-estimate of the deformation and strain compared with those of the actual aircraft structures.
(3) Unlike th e o val shaped win dows in a ircraft to day, the windows were alm ost rectangular, with small
radius a t ea ch corner. C onsequently, the stress c oncentration fac tor w as hi gh, and fa tigue c racks
initiated readily.

2

Failure Knowledge Database / 100 Selected Cases

5. Immediate Action
BOAC stop ped operation of a ll Comet a ircraft immediately, a nd t he British A viation Au thority
suspended the ty pe certificate of t he aircraft. T he Britis h g overnment or dered t he R oyal AircraftEstablishment to conduct a thorough investigation into the cause of the accidents.
Many fragm ents of t he airc raft w ere retr ieved from the sea at a depth of 1 80m by sal vage a nd werereconstructed on the gro und. In p arallel to t hese operations, a pressurizing fat igue test f or the full-sc ale
fuselage structure was conducted by immersing it in a huge water chamber.
6. CountermeasureThorough rev isions w ere made t o the f atigue test m ethodology f or full-sc ale st ructures a nd fatigue
design concept. The major revisions are as follows:(1) For the development of new aircraft, two full-scale structures must be provided for testing: one is forthe static load test, and the other is for the fatigue test including the pressurization test.
(2) In c onducting t he ful l-scale fati gue tests, i n addition to the ef fect o f th e m agnitude and number offatigue loads, the effect of the load sequence must be taken into consideration.
(3) Stress con centration fac tors of cuto uts a nd n otches m ust be c orrectly eval uated, and t hey must b eminimized in the design of the structure as much as possible.
7. Knowledge(1) Effect of proof pressure test: Proof pressure loading is beneficial for the safety assurance of most ofpressure vessels when they are provi ded for service. However, in the case of aircraft, proof press ure
load is not applied to the actual fuselage structure. In such a case, the fatigue life will be much shorter
than that with proof loading beforehand.
(2) Effect of load seq uence on fat igue li fe: In th e ev ent of th e fa tigue l ife ev aluation o f st ructuressubjected to random fatigue loads, the application order of very high overload that might appear only afew times during entire life significantly affects the total life estimate. When a large tensile overload isapplied i n earlier sta ges of the fat igue t est, the total fat igue life b ecomes lon ger, whic h gives us anunsafe estimation of t he actual fati gue l ife. This effect must be t aken i nto ac count for t esting andevaluation.
(3) Notch effects on fatigue life: Stress concentrated by notches often causes fatigue cracks to occur. The
crack initiation time becomes shorter as the stress concentration becomes higher. Therefore, the stressconcentration at notch corners should be reduced by giving the notch corner a larger radius.
8. BackgroundOne year before the accident, on May 2nd, 1953, an accident on Comet occurred at Calcutta, the causeof which is now believed to be the same as above-mentioned in that accident, forty-eight people on board
were killed in the crash that occurred eight minutes after the take-off. At that time, the cause was thought tobe either a p ilot error , a sa ndstorm or a whirlwind. H owever, t he method for investigating t he c ause ofaircraft accidents was not established yet then, and no further examination was conducted.
3

Failure Knowledge Database / 100 Selected Cases

9. SequelAt the t ime of the accident investigation, the ef fect of pressure overloads on fatigue life had no t beencorrectly recognized. Today, however, the effect of overloads on e xtension of fatigue life is considered asfollows:
Figure 6 sh ows the rel ationship between t he cra ck le ngth (a) a nd number of fat igue cycles (N ) for aconstant amplitude fatigue test with periodical overload cycles as given in the Figure. The fatigue life forcase C is four times longer than that for case A as a result of the residual compressive stress induced by thetensile overload. The mechanism for this effect is explained by (a) and (b) in the same Figure. It can also beexplained by the concept of crack closure and plastic wake model.
As described above, the extension of fatigue life by the periodical application of tensile overload has
been verified by theory and experiment.
10. On the Side
While th e U K was inv olved i n t he investigation of t he a ccident c ause and t he development of ne w
safety measures during which time the ty pe certificate of the de Haviland Comet was suspe nded, Boeing
succeeded in developing the B-707 jet air transport aircraft.
By the time that de Haviland had overcome all of the troubles and finished the development of the newComet-Ⅳ, the entire international market of jet aircraft had shifted to the US.

11. Social Impact
This a ccident ga ve m any peo ple t he impression t hat th e c ommercialization of the je t a ircraft w ith
pressurized cabin at that time may be premature. In particular, people became uneasy about its reliability
and safety.
12. Information Source(1) ICAO Aircraft Accident Digest Vol. 6-2(1956) pp.16-45.
13. Primary Scenario
01. Ignorance
02. Insufficient Knowledge
03. Error of Evaluation/Test Method
04. Insufficient Analysis or Research05. Insufficient Prior Research
06. Ignorance of Residual Compressive Stress Effect
07. Insufficient Analysis or Research
08. Insufficient Environment Study
09. Under-Estimation of Stress/Strain
10. Planning and Design
4Failure Knowledge Database / 100 Selected Cases

11. Poor Planning
12. Non-avoidance of Stress Concentration
13. Usage
14. Operation/Use
15. Operation of aircraft
16. Failure
17. Fracture/Damage
18. Fatigue Failure
19. Failure
20. Large-Scale Damage
21. Mid-air Explosion
22. Crash
23. Loss to Organization
24. Economic Loss
25. Loss of International Market
26. Damage to Society
27. Change in Perception
28. Increase of Distrust to Jet Air Transport

5Failure Knowledge Database / 100 Selected Cases

ADF(Automatic Direction Finder)Wing
Aft fuselage

Wing
Fore Fuselage

Fig. 1 Comet-1 (G-ALYP).

6

Failure Knowledge Database / 100 Selected Cases

Load

Gust

Storm

Load

FlightTake-off
Taxiing
Landing

Actual
200,000

2,000

100

2,000

2Time

Simplified

PressureP

FlightP
Landing

TimeFig. 2

Typical load applied to transport aircraft.

7PFailure Knowledge Database / 100 Selected Cases

Crack

Internal pressure (atm)

P1.12

Proof pressure

crack
N=18,0000.56Number of cycles N(cycles)

Internal pressure (atm)(a) Development test
crack
N=3,060
1,230
Normal pressure
0.75
0.56

1,830
Normal pressure
1,000Proof pressure
830

Number of cycles N(cycles)(b) Effect of magnitude of proof pressure

Fig. 3Pressurizing Pattern of CometⅠ

8

Failure Knowledge Database / 100 Selected Cases

LoadedUnloaded
Rivet
Compressive
residual stress field

Small radius high stressconcentration

When unloaded, surrounding elastic
zone induces compressive residual
stress to plastically deformed areaby spring backTensile plastic
deformation area

Fig. 4

Deformation at Window Corner by Loading and Unloading of Overload.

9

Failure Knowledge Database / 100 Selected Cases

40

Crack length a (mm)

CA B

30
202(kg/mm )10ASm=8.2 Sa=3.3

8B

Overload Smax=19.2
Smin=-2.964

Overload Smax=19.2

C
100

0200

300

400500

Number of cycles (×10 )3σyσyσys
Stress distribution+rr
Residual stressdistribution(a) Overloaded

Fig. 5(b) Unloaded

Relation Between Crack Length and Fatigue Cycles Under Periodical Overloads.

10