Boeing’s 737 is not an airplane many would consider big when seen next to its larger 777 or 747 stablemates, but it carries a huge amount of energy to the runway on every approach. As you may or may not have noticed, Boeing has stretched the 737 design so much that its -900ER model now holds as many passengers as a widebody 767-200. To carry all that weight to the runway, the long-body 737s (-800 and -900 series) have very high approach speeds of 155 to 165 knots depending on flap selection. For a comparison, picture yourself at the cruise speed of a Mooney as you touch down.
So here’s 157,000 pounds of airplane approaching the runway at nearly 190 mph. How do you bring all that energy to a crawl? Thankfully, there are several systems to do the job—lift-destroying ground spoilers, thrust reversers, and some seriously heavy-duty brakes. Unlike the very basic brakes in light airplanes, these are gigantic and employ antiskid and autobraking systems. In combination, these features can bring the above-mentioned jet to a stop in about 2,400 feet of dry runway after touchdown. That’s some serious seatbelt-straining power.
On dry runways, the most important systems needed to stop the jet are the spoilers and the brakes. The lift-killing spoiler system is the unrecognized hero that does much of the work in slowing the airplane. Without the weight transfer from wing to wheels that the spoilers supply, the brakes can’t do their job.
Many pilots use full reverse thrust on dry-runway landings, but in the case of new 737s, it mostly just makes a lot of noise and burns a lot of gas for very little stopping power. If the runway is long and dry, and the airplane landed in the touchdown zone, reverse is usually unnecessary in the 737. The landing-distance charts illustrate this quite well. Using 30 degrees of flaps in the -900ER, reverse thrust only reduces the ground roll 90 feet during a max-weight landing.
My technique on a dry runway is to select reverse thrust but leave the engines at idle. Selecting reverse in the 737 translates the cowl of the CFM engines rearward to divert the thrust from the huge N1 fan at the front of the engine. This stops the big N1 fan from producing any idle thrust, and it diverts the fan’s thrust diagonally forward. It’s a noticeable drop- off in residual thrust when the reversers open up.
Other jet engine designs use a “bucket” system in which clamshell-like buckets at the rear of the engine divert all thrust forward. There are also thrust attenuators that are employed on the Cessna CitationJet, for example, that are not reversers per se. They simply block engine exhaust to minimize the residual idle thrust—something that we don’t need when trying to stop. Finally, there’s the lift-dump system used on Hawker jets. This system extends the flaps to 75 degrees on touchdown in combination with spoilers.
Many safeguards are employed to keep a reverser from opening up at the wrong time. Weight-on-wheel switches and interlocks on the reverse levers are employed to be sure that reverse is not being selected in flight. If a reverser fails to deploy, a combination of lights will illuminate to alert the pilot that one or both have failed to open. If only one fails, the pilot must be careful to limit reverse thrust on the operating engine to avoid yaw during the rollout.
Spoilers also employ similar logic to avoid inadvertent deployments at the wrong time. Most high-end jets use a combination of weight-on-wheels switches, radar altimeter input, and wheel-spin sensors to allow the ground spoilers to deploy. And when those spoilers pop up, it’s quite obvious as the airplane firmly squats down on the runway. Unlike small general aviation airplanes, jets are not at all “light on the wheels” just after landing.
Thanks to those spoilers, the brakes on jets are very effective immediately after touchdown. This is where autobrakes earn their keep on every touchdown. These systems combine with antiskid braking (just like ABS in your car) to apply the maximum amount of braking based on the amount of weight on the wheels and the actual traction that you have. As the pilot is still “flying” the nosewheel down to the runway, the autobrakes are already slowing the jet. Try that in a light airplane and you’ll quickly smoke your tires. Once the nose is down, the pilot can apply manual braking, which automatically kicks off the autobrake system. In the 737, stowing the spoilers also kicks off the autobrakes.
When runway conditions are slick, the thrust reversers begin to pay off. Since traction is limited, autobrakes can’t do their job nearly as effectively. In these cases, the thrust reversers—and to a far lesser degree, aerodynamic braking with the flight controls—pick up the slack. The landing-distance charts show that full reverse thrust on runways with poor braking action can reduce landing distance by 2,800 feet. A couple of warnings about reversers come into play, however. Ingestion of foreign objects into the engines is more likely as the airplane slows. It’s important to stow the reversers as the airplane slows below about 60 to 80 knots, depending on the airplane. Likewise, blasting thrust forward can create limited visibility on snowy or dusty runways—another reason to stow them before getting too slow.
One note about airplane design and braking is the use of “double-bogey” wheel sets. A good comparison example is the Boeing 757 and the 737 designs. While the long-body 737s can haul as many people as a 757-200, it has a much harder time stopping in part because it only has a single set of wheels on each main gear. The 757’s double-bogey setup provides much better stopping power. Naturally, the more rubber on the pavement, the better the traction to stop.
Pete Bedell is a Boeing 737 first officer for a major airline and co-owner of a Cessna 172 and Beechcraft Baron.
Simply simple systems
By: Mark R. Twombly
A turbine-powered airplane is a collection of complex systems and intricate moving parts, so it’s refreshing to know that an absolutely critical function—getting fuel out of the tanks and to the engines—is accomplished by a simple system that relies on a component with no moving parts. That component is a jet or ejector pump.
Why call it a jet pump? If the operating principle behind a jet engine can be described as suck, squeeze, bang, and blow, then a jet pump can be said to squeeze, expand, suck, and blow.
A jet or ejector pump is part of the fuel-distribution system, which continuously transfers fuel from the tanks to the engines. The fuel-distribution system is designed to supply more fuel to the engines than they will need under any operating circumstances.
Fuel from the tank to the engine passes through an engine-driven pump, which feeds some to the engine and the rest to a bypass line that leads back to the associated fuel tank. This so-called motive flow fuel goes directly to the primary jet pump in each tank. The high-pressure motive flow fuel is forced through a small orifice inside the jet pump, and as it exits the orifice the pressure drops.
The now low-pressure fuel passes across an inlet that is open to the tank sump area, and the low pressure causes a large amount of fuel from the sump to be sucked through the inlet into the jet pump. From there the fuel exits the pump to a line that connects to the engine-driven fuel pump.
Thus, the fuel makes a continuous loop from jet pump to engine-driven pump and back, passing through various filters and valves on the journey. The system is self-sustaining as long as the primary jet pump continues to receive motive flow fuel from the engine-driven pump.
Most turbine fuel systems also incorporate secondary or transfer jet pumps in the fuel tanks. These serve to transfer fuel from various locations in the tank to the sump or engine feed reservoir where the primary jet pump is located. The secondary jet pumps use motive flow fuel from the primary jet pump’s output.
Using motive flow fuel to “power” primary and secondary jet pumps is a beautifully simple way to transfer fuel from the tanks to the engines without relying on yet another collection of complex, expensive moving parts.
But as simple as a jet pump is, stuff happens and you have to be prepared with a Plan B. In this case Plan B is an electric boost pump located next to the primary jet pump. If fuel pressure drops, the boost pump takes over to restore pressure in the system.