ISTR hearing a long time ago that the landing gear of commercial aircraft were not supposed to be deployed above a certain airspeed, lest they be damaged by excessive aerodynamic forces.
But somehow this doesn’t add up. Consider, for example, that a 747’s undercarriage is designed to tolerate hundreds of thousands of pounds of braking force, I’m skeptical as to whether damage is the real concern.
Pilots, what’s the deal? Is there an airspeed above which the landing gear are not supposed to be lowered? If so, what’s the reason? Potential gear damage? Excessive pitch-down behavior due to aero drag on the gear? Something else?
Perhaps the concern is more for the landing gear bay doors, which are not designed for that much force and could be ripped off if deployed at too high speed?
They are designed to bear the weight of the vehicle and designed to tolerate braking loads, the latter being front-to-rear forces applied to the tire at the contact patch, and transferred by the wheel’s hub to the axle, and…on up to the points where the landing gear are affixed to the aircraft.
IOW, yes, they very much are suitable for massive shear loads in the range of tens or hundreds of thousands of pounds.
I’m just wondering what’s so different about braking loads versus aerodynamic loads at high airspeed. I can imagine floppy things like brake lines or bay doors possibly fluttering to the point of self-destruction in the slipstream. Is that the reason, or is there something else?
I bet it has something to do with the hydraulics that raise and lower the gear.
Put them down at too high of a speed, and you likely run a risk of seriously damaging the hydraulics that raise and lower the gear, more than you probably risk damaging the actual gear itself.
Well, when the gear doors get all bent & stuff, it usually jams at least one gear leg and then the hydraulics don’t often break, they do the breaking of other stuff like bell cranks and pivot points and then there sits this pilot with a floppy nose gear waving in the breeze and many bad choices are my lot.
Bomb bay doors were made so as to be opened at speed but civil aircraft builders do not usually spend that kind of money building those parts and adding the cost and weight to a design.
Their aircraft cost too much already and that is bad. Adding weight = bad also.
Some aircraft that have gear doors that re close once the gear is out can fly at a higher speed under that condition. In transit they need a lower speed.
One thing I learned from doing a logistics project tangentially related to airplane maintenance is that you can still fly a 737 from one airport to another when its landing gear won’t retract, (e.g. from one where there is no crew/equipment to fix landing gear retraction to one where there is) but the fuel economy is horrible.
that would lend support to AndrewL’s posit that it has to do with the doors, since the 737 doesn’t have any. The gear tucks into the under side of the fuselage.
Some planes can tolerate very high gear extension speeds. In 1979 TWA flight 841 (a Boeing 727) exceeded Mach 1 in a dive, and the pilot eventually extended the landing gear at 470 knots (540 mph). It damaged the landing gear but the drag helped the plane recover and it landed safely.
Note there is a difference between maximum airspeed to extend landing gear vs max allowed airspeed with gear down and locked. E.g, the 737’s max speed with gear down is 320 knots, but maximum gear extension speed is only 270 knots. Furthermore maximum gear retraction airspeed is only 235 knots.
When the gear is in transit it is not locked and various hydraulic and mechanical factors limit max airspeed the gear can withstand. Imagine a forward-deploying nose gear, where hydraulics must push it forward into the windstream. Now imagine that nose gear retracting (which seems easier) but the rate must be maintained within certain limits. Hydraulic rate valves or even bungee mechanisms are only designed for a certain range of forces for gear in transit. Air drag increases as the square of velocity so the gear doors or gear itself can build up very high forces.
If the gear is down and locked it can typically take higher forces, e.g, if the plane took off, never retracted the gear and kept accelerating. But even then if you calculate the drag force on various landing gear (inc’l doors) at high airspeed, it will be very high. It’s true the shear forces on the gear under heavy braking are high and it endures that. I am guessing the drag force at high airspeed (being a squared term) is even higher.
You can fly any aeroplane with its gear extended, you just need to respect the limiting airspeed, and yes the fuel economy is terrible.
I don’t have much to add to what has already been said. At some point in the retraction/extension cycle, doors are hanging down (even on a B737) and those doors can’t take the same air load as the rest of the plane. There may be some other weak point, but doors is the normal one.
you also need to respect limits such as geography. Some airports are in valleys and the landing gear could cause a reduction in climb rate that is enough that it then leads to a requirement to bank, leading to a negative climb rate… which means they are trapped in the valley. In some situations, it could be too late to safely bank …to transit the valley in the other direction. But they were flying …that would be controlled flight into terrain.
As far as I can tell, TWA 841 did not exceed Mach 1. The Wikipedia article on TWA 841 says it exceeded the Mach limit for the airframe, which I take to be MMO, but on a 727 that’s Mach 0.84.
For what it’s worth, I found the Longform article incredibly grating. It’s riddled with basic factual errors (e.g., referring to airspeed not in knots but rather “knots per hour,” a discussion about instrument panels over the passengers’ heads, etc). This combined with the authoritative and patronizing tone just irritated the hell out of me. It bothered me so much that I googled the author’s name.
It turns out Buzz Bissinger is an investigative reporter and a winner of the Pulitzer Prize. He’s also the guy who wrote Friday Night Lights. So clearly he’s got some writing aptitude.
I was shocked to discover that a “real” writer wrote the Longform article. My complaint isn’t exactly that he got technical details wrong…it’s that someone who calls an aircraft yoke a “[steering] wheel” clearly isn’t paying much attention to what the technical people are telling him. I’ve worked as a journalist and my dad was an investigative reporter for decades: getting these details right is generally considered important in journalism. Mark Bowden (Black Hawk Down), for example, would never have made mistakes like this.
The article has an odd straining-for-authority, voiceover tone. That combined with laziness about technical details is what set me off.
I’m not saying that the article was bad—only that it grated on me. And I’m kind of a special snowflake in this regard (journalist turned mechanical engineer).
As has been mentioned, the gear doors may be damaged. Heck they may even “depart the aircraft”. While this in itself is bad, where the go may make things even worse for the pilot(s).
Note, having any part of the aircraft “depart the aircraft” is bad. If it is ingested by an engine, very bad things can be happen. Pilots in general do not like it when their engines quit making thrust while they are in flight.
If the doors are ingested by the engines, they will do damage to the engine(s). How much damage can vary. Any thing from a minor inconvenience such as excessive noise, to complete engine failure can result. Worse case scenario, I suppose, would be if the engine was damaged enough to become so unbalanced that it could rip itself out its own engine mounts. This could result in the engine departing the aircraft. Which in all likelihood would make the airplane uncontrollable. Not good.
Of course some of the fluids would leak out if this happened. Most of these fluids are fairly flammable. Pilots also do not like in-flight fires. At this point, things can get real intense in the cockpit.
It is best to not exceed the maximum Vso, (flaps, gear, speed brakes, etc. extended), that the manufacturer has set forth in the manuals.
And if you are really good, you can shut the engines down, feather if the plane is equipped, (weren’t some DC-2’s and a few early 3’s without a feather ability? ), and position the prop so as to minimize or totally avoid prop/engine damage on smooth hard surfaces?
Every plane I owned or worked with on a regular basis I would do several dead stick ( all engines stopped or feathered ) as to know glide qualities. Too many reports of pilots over flying foam because they had no idea of how good a glider it was without windmilling propeller/s…
( This knowledge actually made a difference a few times when I was experiencing other forms of my bad judgement. )
Not recommended for low time pilots or those that will not get outside the rules or box so that they better avoid bent or ruined aircraft or bodies. ( Just what I did back in the day with the lack of quality aircraft / maintenance I had to fly to be able to eat so I could make it to “Old Pilot” which I have. )
And you can land one gear up and *still *get the weight on the wheels.