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Plane Answers: Glide ratios and the most critical phase of flight
Twenty-four hours before the US Airways ditching in the Hudson River, I received this question from Roger:
I have taken lessons in a 182 Cessna and I remember that the glide ratio was very good if there was ever a need to land without power. What is the Glide Ratio for something like a 767, 747 or an MD80 or [a bag of] rocks? Hopefully, I'll never get to experience it on a commercial airliner in flight.
Tell me you didn't ask this question and then jump on a flight from New York to Charlotte the next day, Roger!
The truth is, on almost every jet airliner, we use a rule-of-thumb that says it will take three miles to descend 1000 feet at idle. That same rule-of-thumb would work for an airliner that has lost all engines.
So at 3,000 feet, you should be able to glide 9 miles. At 30,000 feet you'll be able to glide 90 miles. And here's an interesting twist. A heavy airliner will actually glide farther than a light one because of the added momentum.
This glide ratio is at least 16:1 or 16 feet forward for every foot of altitude lost. This is a ratio right up there with a Schweizer 1-26 glider, and better than most birds or a hang glider in fact.
As for the bag of rocks, well, I'd even prefer to be in an MD-80 than a 'dead-stick' bag of rocks. (pilot humor, excuse me folks).
Kylie asks a question, again just hours before the accident:
I'm not sure that the reason I'm so nervous about flying is that I'm not in control, but that I don't know how "in control" the pilots are if something goes wrong. One of my friends told me that one time he was on a plane and all of a sudden the power goes out, it sounded like the engines stopped, the plane seemed to slow down, and began to drop. However this took place over a few seconds before everything came back on. I'm always worried that if some unexpected problem occurred, and something caused the engine(s) to stop working, is there anything that pilots could do, or is there any back up that would get the engines to work again?
The FAA requires that all transport category aircraft have enough performance to operate on one engine in all phases of flight including just after takeoff. When deciding on the criteria for a two-engine aircraft to cross the Atlantic, they did some studies to calculate the odds of both engines failing. These odds were overwhelmingly small, since each engine is built to have a record of no more than one failure per 30,000 hours or so. So the odds for both engines failing is exceedingly rare, even with the recent bird strike accident.
There have been cases of fuel exhaustion, most recently with an Air Transat A330 that developed a significant fuel leak. That airplane successfully glided to the Azores off the coast of Portugal for a rather successful landing at the airport. The relatively good glide ratio shown above allowed them to travel nearly 100 miles without power.
We also have an extra generator not associated with the engines called the APU that's available if we need it. This generator can power the entire airplane on its own. Finally, if we lost all three sources of power, on the 757 and 767 that I fly, there's a small wind generated turbine called a RAT (ram air turbine) that pops out below the airplane and drives a generator to provide power to the flight controls. So double and triple-redundancies are built into every airliner.
Rose asks an equally dramatic question:
I have a brother who is an aeronautical engineer, and he tells me that the first 15 seconds of a flight, (once the wheels have left the ground) is the absolute most volatile because there is no recovery. If something 'goes wrong'....the aircraft "must fly" before it can land. So he always tells me to count "one-one thousand, two - one thousand....until I reach 15. He says that if I'm 'still alive', that my chances of survival increase by 1000%. Is this true?
We actually call it a critical phase of flight. And that's why the FAA makes us train for just about any scenario during that period.
Fortunately, modern airliners are rather overpowered and an engine failure is rather easily overcome by the thrust of the other engine. I've actually flown many small planes that didn't have the performance with all engines running that a 757 does on one engine.
Ahh, but what about the dreaded dual-engine failure caused by geese you're surely asking. There have been a number of engine failures that have been caused by a bird strike, but it's incredibly rare for both engines to be taken out.
Contrary to what I've heard reported by some in the media, we do practice two-engine flame outs, at least at my airline. We usually save it as a 'bonus' practice at the end of our checkride or training.
Looking at this graph below, the portion after liftoff until the flaps are all the way up represents 5% of all accidents and 14% of the fatalities, interestingly.
Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he'll use it for next Monday's Plane Answers. Check out his other blog, Cockpit Chronicles and travel along with him at work.
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