Bombs Away?Explosions aboard Commercial Aircraft

Arnold Barnett

MIT, LFM

No rational person would say that we should pay any price, however enormous, to achieve any increase in security, however minuscule.

Still, some recent US security decisions are hard to understand.

Why is air cargo-- very little of which is screened for explosives --transported aboard US passenger jets?

If cargo were banned from passenger jets, the profit that the airlines get from transporting freight could be covered by a $2 “user fee” per passenger.

Suppose that air travelers were asked:

“Would you be willing to pay $2 more per trip to travel in a jet that is not carrying cargo unscreened for explosives?”

What do you think the answer would be?

Why did they end positive passenger bag match (PPBM) when explosives detectors were introduced to inspect checked luggage?

Absent PPBM, the explosives detector becomes something of a roulette wheel, which it costs almost nothing to play and with a chance of winning equal to the false negative rate of the machine (which is not zero).

When US carriers performed PPBM in 2002:

• 1.2% of departures were delayed because of the policy

• Among those flights that were affected, the mean departure delay was 10 minutes.

• The dollar costs associated with the policy averaged about 10 cents per boarding passenger.

But is PPBM no longer cost-effective?

Things have changed recently, because of the 25% increase in the amount of checked luggage on US domestic flights.

This increase in checked luggage would be expected to have a larger-than-linear effect on bag-match delays:

• One might expect a 25% rise in the probability a bag-pull would be needed.

• Because there are 25% more bags in the luggage hold, the time to extract a bag might grow by roughly 25%.

Moreover, the doubling of fuel prices since 2002 presumably has a comparable effect on the dollar cost per minute of departure delay at the gate.

Revised PPBM Statistics:

•Probability of Delay Grows from 1.2% to 1.5%.

• Mean PPBM Delay Given that One Occurs Grows from 10 minutes to 13 minutes.

• Mean dollar cost of PPBM per passenger boarding grows from 10 to 15 cents.

Are we willing to pay this price to keep unaccompanied luggage off originating flights?

(Under certain ATA statistics, PPBM would be cost-effective if it prevented one successful bombing every 50 years.)

Proposals for new measures to prevent airborne explosions are popping up everywhere.

Separating people from their laptops would be painful, although some people could surely use the time to go over reading material, or even revert to pen and paper.

…For now, the surest way to keep dangerous materials out of the cabin is to keep virtually all materials out of the cabin.

The New York Times, 9/10/06

Are they crazy?

Are we doing enough emergency planning to prevent on-board explosions that might be imminent?

Suppose that a jet plane has suddenly plunged from the sky, for reasons unknown.

What should we do, knowing that terrorist acts against transportation often are followed by others within minutes?

One possibility:

Immediately order all planes above 10,000 feet to descend to that altitude, where they would have a reasonable chance of surviving an on-board explosion (better than 50/50).

Calculations and Common Sense Make Clear Some Realities:

• From the moment of the first explosion to the issuance of the “descend immediately” command, at least three minutes would elapse.

• If a jet at 30,000 feet received such a command and began descending immediately at the maximum safe angle (15º, according to Air Transport Association), it would require two minutes to reach 10,000 feet.

Thus, a minimum of five minutes would elapse between the initial event and the arrivals of endangered jets at a “reasonably safe” altitude.

Major Fatal Terrorist Bombings Against Air/Rail Since 9/11

Year Location Number of Attacks2003 Madrid 10 within 3 minutes2004 Moscow One2004 Russia One2004 Russia 2 within 2 minutes2005 London 3 within 1 minute2006 Mumbai 7 within 11 minutes

In other words:

Based on the degree of simultaneity in these six events, perhaps two of the 18 follow-up bombings could have been thwarted in the sky, given a 5-minute response time. This works out to an average of 0.33 jets saved per event.

But, of course, there is collision risk when large numbers of planes descend at once towards the same altitude.

Calculations based on geometrical probability indicate that, if a jet undertook an unannounced emergency descent from 36,000 feet to 10,000 feet without changing direction, the chance it would collide with some other plane at some lower altitude could sometimes approach 1 in 100 (!).

Even assuming some strategy under which planes at differing altitudes descended at different angles:

It is hard to see how average loss to collisions would fall below 0.5 jets per alarm. And some of the alarms that led to “descend immediately” commands would be false.

Tragically, therefore, the “optimal strategy” given the sudden loss of a jet might well be:

Do nothing….except hope.

Why are we going ahead with a Trusted Travelers program?

Under the original plan, Trusted Travelers would submit to background checks and biometric processing. In return, they would get expedited screening and less stringent physical security at airports.

As a practical matter, the only applicants for trusted traveler status who would be denied it would be those who lied flagrantly on their applications or were on terrorist watch lists.

Indeed, in the “trial run” program now in use at Orlando, 23,000 people have applied for Trusted Traveler status. “You can count on both hands” the number of applicants who have been rejected.

New York Times, May 16, 2006

Thus, sleeper terrorists (like the London Underground bombers) who successfully attained trusted-traveler status could come to the airport guaranteed in advance that their security screening will be limited.

The risks inherent in the trusted-traveler program are especially unfortunate because its main objectives could be met without any reduction in security.

TSA has apparently become much more reasonable on this issue recently. But, under its latest stated position, the biometrics and the background checks have no logical rationale.

So, where are we?