Tracking Small Planes with GPS

The last quarter-mile of Corey Lidle’s fatal flight in Manhattan went unrecorded by radar. GPS technology could allow more precise accident reconstruction.

In the wake of the small-plane crash into a Manhattan apartment building last week, politicians and the public have focused on the potential dangers of unmonitored small-plane traffic around New York City.

But a technology question arises: Why don’t these planes–which aren’t required to carry “black box” voice and data recorders–at least required to save their GPS position information, for more accurate accident reconstruction?

The plane that crashed, killing N.Y. Yankees pitcher Corey Lidle and flight instructor Tyler Stanger, was last seen by conventional radar units about a quarter-mile away from the accident site, in the middle of a U-turn, at an altitude of about 500 feet, according to the National Transportation Safety Board. That leaves a lot of data missing about what happened in the final plunge before it exploded against the building.

Inexpensive equipment could fill that void.

John Hansman, professor of aeronautics at MIT and director of MIT’s International Center for Air Transportation, discussed this and other crash-related issues with Technology Review.

Technology Review: What is the difference between radar and GPS in terms of the ability to record flight data?

John Hansman: Radar registers a plane’s position every time the radar sweeps by, which is typically [every] four seconds. GPS typically calculates coordinates once per second. And at low altitudes, radar can be shielded by tall buildings. Radar doesn’t actually measure altitude–it measures position–and the plane responds by reporting its altitude to the nearest 100 feet. GPS data on the plane can be accurate to a few feet. And it can be stored in the GPS unit on the plane. There is also an emerging system called Automatic Dependant Surveillance-Broadcast (ADS-B) that would allow the GPS data to be broadcast and received by air-traffic controllers.

TR: What’s the significance of that radar/GPS difference in terms of this crash?

JH: With GPS, you’d be able to better reconstruct what actually happened. Was he descending? Was he descending at a rate that was equivalent to having lost an engine? Was he turning? There’s a lot of stuff they will get out of the forensics–how the propeller bends on impact depends on whether the plane’s engine was working. But from GPS you could get more information on how the airplane was maneuvering prior to impact.

TR: Did the plane that crashed, a Cirrus SR20, carry any GPS equipment?

JH: Yes, it had a GPS that fed data into a multifunction moving map display (MFD). GPS trajectory storage is a function included in some GPS units. I am not sure what unit was on this aircraft, so it is hard to say exactly whether it had a GPS record. Apparently, in the NTSB briefing they did indicate that they had recovered a memory chip off the MFD. But I don’t know what data was recorded on the chip and whether the data would have been wiped out during the crash. There is no requirement for flight-data record for general aviation, but investigators will take advantage of any evidence they can get.

TR: Do you think the FAA should require small planes to record their GPS data? It wouldn’t be so expensive, would it?

JH: Take your garden-variety, hand-held GPS unit, and it can show you the track of where you have been. It doesn’t take a lot of memory to record the coordinates of recent flights of an airplane. It has become very common to archive the trajectory data in memory. We do this sort of thing with cars now. We are naturally moving into an era where most accidents will be more accurately diagnosed with data residing in electronic systems.

What is happening now is there is a de facto move to have that capability onboard, because it is so cheap. However, if you make it a requirement, it would get complicated. You would require not only the memory, but some level of protection so the memory doesn’t get burned, so it’s survivable.

TR: As you alluded to earlier, future air-traffic control could be based on planes beaming out their GPS data, a technology known as ADS-B, that will join–and in some cases replace–radar-based monitoring. Will this help?

JH: The FAA is requiring ADS-B in the next 5 or 6 years, which will allow transmission of position and other aircraft states (such as velocity). This will initially be for commercial passenger aircraft, but the expectation is that it will be common for all aircraft. This will become part of the FAA’s surveillance system. Examinations of radar tapes would be supplemented with GPS-based ADS-B data, which will be higher resolution and have a faster update. You will then still need an antenna in range to receive the information. If you don’t, or if it’s blocked by a building, you would still need that data onboard the plane if you wanted to recover this information after a crash.

TR: This crash has led to a political drumbeat for tighter restrictions on small planes around Manhattan, because of the apparent terrorism risk. What’s your take on that?

JH: The interesting thing is that this event shows that small airplanes are not threats to buildings. There’s all this talk about regulating planes more; but what this shows is that if you take a fully loaded small airplane and fly it into a building: the airplane bounces off the building. They don’t have enough energy to penetrate and challenge the structural integrity of the building.

TR: This plane included a parachute designed to support the entire plane in an emergency. Could it have saved lives in this situation?

JH: The parachute is interesting, but it doesn’t protect you from everything; at low altitudes it does not work because there is not enough time for deployment. It’s not clear that at 500 feet or below they could have deployed it reliably.

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