Friends have been asking me about the crash of N121JM, a 2000 Gulfstream G-IV, after running off the end of Runway 11 at Hanscom Field on Saturday evening (Boston Globe). I didn’t know any of the people involved, I don’t have a Gulfstream type rating, and at this point the most that anyone can do is speculate. However I will share with readers what I have shared with friends, mostly based on my experience flying the Canadair Regional Jet, a similar size aircraft.
Background: In theory, you take off in a twin-engine turbojet by holding the brakes, pushing the thrust levers forward, verifying that you’ve reached full power, and letting go of the brakes. The pilot flying looks down the runway. The pilot monitoring checks the airspeed indicators on both sides and says “100 knots cross check”. Then the pilot monitoring calls out “V1“. This is the “decision speed”. If an engine quits before V1, you pull the thrust levers back, hit the brakes and stop before running off the end of the runway. The FAA allows about one second as a reaction time and assumes near-perfect technique after that. This is the reason that commercial airport runways are so long. The plane needs enough runway to come within 1 millisecond of taking off and then enough runway to brake to a stop from 150 miles per hour or so. If an engine quits and you’ve reached V1 you continue the takeoff on one engine. You wouldn’t be at that airport with that load of passengers and fuel if the dispatchers hadn’t calculated your ability to take off and climb out to clear obstacles on one engine (i.e., you won’t be able to carry as much weight if taking off from a high altitude airport surrounded by mountains, since turbojet engine power output falls as altitude increases).
Takeoff configuration: The CRJ simply would not take off, even from a 15,000′ runway and with full power, unless flaps were extended. The clean wing was designed to minimize drag during high-speed cruising. Thus it is critical to have the flaps properly configured for takeoff or the airplane will simply keep accelerating down the runway without lifting. All of the standard performance charts for the Gulfstream G-IV assume “flaps 20″ but the NTSB reports so far have described the flaps being set to just 10 degrees in the cockpit (given that the plane supposedly reached 165 knots, it should still have been able to lift off at flaps 10).
Refinement 1: In the real world, pilots aren’t that great at aborting takeoffs and it is hard on the airplane’s systems to slam to a stop once the plane reaches about 115 miles per hour. So at our airline we had a rule that unless it was something pretty dire we would continue the takeoff once we reached 100 knots and then work out the problem in the air.
Refinement 2: Passengers prefer a “rolling takeoff” in which the brakes are not held as the thrust levers are advanced. Unless one is operating from a short runway in the mountains, this is how take-offs are typically done. Unfortunately this chews up additional runway due to the plane rolling as the engines “spool up” to full power. For the CRJ, no data are available regarding exactly how much runway is wasted in this fashion and whether or not the plane can still be stopped after recognizing a problem at V1. A friend sent me a portion of the Gulfstream G-IV Airplane Flight Manual (AFM) and it seems to indicate that the performance charts can be relied upon even given a rolling takeoff.
Refinement 3: In the real world, a twin-engine turbojet has way more power than it needs for most flights. If you’re lightly loaded (few passengers, short trip so not too much fuel) and departing from sea level, why would you want to make those engines work so hard? The FAA Advisory Circular AC 25-13 from 1988 explains that “Takeoff operations conducted at thrust (power) settings less than the maximum takeoff thrust (power) available may provide substantial benefits in terms of engine reliability, maintenance, and operating costs.” An additional advantage of reduced thrust is that passengers on a lightly loaded plane won’t be slammed back in their seats like astronauts.
If you’re an engineer you would naturally assume that this would all be idiot-proof on a $50 million plane stuffed full of computers. The airplane infers its departure runway and airport altitude from the GPS location and heading. You push a button for “reduced thrust”, the airplane reads its weight from the strain gauges on the landing gear, and then you advance the thrust levers fully when you’re ready to go. The airplane will make sure that the flaps are set properly and if one engine stops developing thrust the other one will automatically advance to full power. If you lived to be 1000 years old you would probably not be able to get this design certified by the FAA (for the same reasons that the FAA-run air traffic control system will not send your airplane a text message with the instrument flight plan waypoints; instead a controller will read it to the pilot over the radio and the pilot will enter a bunch of 5-letter waypoints and 3-letter VORs into a GPS (possibly getting them wrong)).
In real life what happens is that the pilots calculate the aircraft weight (we did it on paper back in 2008!) and then use a paper chart or maybe an iPad app to calculate the proper reduced thrust setting. If there is a error in this calculation or the transcription from the calculation or the entry of the airport/runway, the resulting thrust might not be enough to become airborne. This is a serious problem because pilots are making the go-no-go decision primarily on aircraft speed (V1 yet?) not based on how much runway is left. The assumption is that the calculations have ensured that the runway length will be sufficient for all possible events. And of course the actual setting of the thrust requires the pilots to watch gauges, another opportunity for misinterpretation. The airplane, though equipped with a GPS, does not have a warning such as “You’ve got 2000′ of runway left. Maybe it is time to go to full power?”
Can a pilot get this wrong? Sure. In fact, four pilots can get this wrong, as demonstrated on March 20, 2009 by Emirates A345, an Airbus A340-500 departing from Melbourne (official report). A simple data entry error caused a lower-than-sufficient thrust to be calculated and the $200 million airliner ran off the end of the 12,000′ runway, taking out lights and antennae. The Emirates crew made the “go” decision rather than the “stop” decision of the Hanscom Gulfstream. When the end of the runway was near the pilot pushed the thrust levers forward for full power and the airplane then flew quite easily.
Please don’t read this posting and infer that I know anything about why this Gulfstream crashed. The intent is just to answer the question that friends asked repeatedly, i.e., “Is there any way to crash a modern business jet on takeoff without the cause being a catastrophic mechanical failure?” And I am as saddened as everyone else about the loss of life.
[Separately, local pilots have been discussing the safety record of our airport, which has more than 150,000 operations per year. The NTSB database shows that the most recent fatal accidents were the following:
- 11 years ago: four-seat single-engine Cessna crashed on an instrument approach due to pilot disorientation in the 400′ overcast (report from 2003).
- 16 years ago in a four-seat single-engine Piper that got slow in a turn and suffered an aerodynamic stall after a 4.5-hour flight (see this NTSB report from 1998)
- 30 years ago: four-seat single-engine Piper crashed due to spatial disorientation on an ILS, almost identical to the 2003 accident (report from 1984)