center line thrust?

[Ryan]

Im really bored at work right now and was thinking of airplanes when this question popped up.

Ive read that most military jets are considered center line thrust. I was just wondering if planes such as the 727 and dc-9 were also considered center line thrust or am I missing what centerline thrust actually is?

Thanks

Ryan

 

[CRJ200FO]

Although the thrust generated from aircraft such as the DC-9 and MD-88 are closer to the centerline of the aircraft, they are not really centerline thrust. Aircraft that have one engine mounted on the nose, and another on the rear of the fuselage lined up exactly with the centerline are the only real centerline thrust aircraft. If a DC-9 were to experience an engine failure, it would yaw towards the failed engine. This would not happen on a real centerline thrust aircraft. Hence the restriction placed on multi-engine ratings received with centerline thrust aircraft.

 

[boxcar]

For FAA purposes, a multiengine airplane with a minimum stall speed higher than Vmca is considered to be centerline thrust. The location of the engines is not a consideration. The DC-9 and B-727 are not centerline thrust airplanes.

 

[CRJ200FO]

For FAA purposes, a multiengine airplane with a minimum stall speed higher than Vmca is considered to be centerline thrust. The location of the engines is not a consideration. The DC-9 and B-727 are not centerline thrust airplanes.

True. I stand corrected.

 

[f18pugsley]

I almost posted something about this on a different thread a while ago, but now you had to ask it.

The FA-18 lands at 8.1 degrees angle of attack (on-speed aoa). On speed ranges from approx 135 KIAS at no gas to almost 185 KIAS at max gross. Single engine minimum controllable airspeed is 10 degrees aoa. Stall speed in landing config is about 15 degrees aoa.

However, the FAA says that the FA-18 is centerline thrust because there is not an 'airspeed' listed. (No kidding--the quote from the FAA rep I spoke to said 'but what airspeed is this?')
Huhhh???!?!?

This is just a sore point with me that I had to fly a Semihole to get the centerline thrust restriction removed. The Seminoles' Vse is ONE KNOT above stall speed at max gross! By the time you burn enough gas to get airborne, you stall before you reach Vse. (That is why you have the '1/2 boot' restriction while training--otherwise you wouldn't ever get the departure!)

I am sure I just ran into a bad FSDO rep, but it really worries me that someone who doesn't understand the correlation between aoa and airspeed in level flight might be determining my license status sometime in the future.

Some ME mil jets are genuinely CL thrust (like the T-2). There are quite a few that are listed as such that shouldn't be (18K lbs thrust 3ft off centerline is a whole lot more moment than 180hp 8ft off centerline.)

 

[flywithruss]

DC-9

I recall thinking that, because the engines were mounted so close to the centerline, that V1 cuts in the -9 would be easy. When I got in to the simulator, I got religion quickly.

The -9 will yaw substantially during an engine-out, and you really do have to feed in a bit of rudder to straighten it out. That said, it flies pretty well engine-out. Unlike training twins, in jets you actually have enough thrust to climb to and maintain an altitude sufficient for a safe return to the airport.

R

 

[TgrPlt]

For your enjoyment and humor, a couple FSDOs that I've had contact with consider the B-1B a centerline thrust aircraft. I know what the FARs say, but after a few demonstrations with 2 engines in full AB and 2 not running I find that hard to believe.

 

[avbug]

Folks,

Bear in mind that when you seek answers at the FSDO level, you're seeking blank information. The FSDO has NO authority to interpret the regulation or make policy. You can ask a question, but the answer you get in return carries no more weight than an opinion.

Seek guidance through FAA publications and higher levels such as the region (for regional questions), etc. Get familiar with the legal interpretations which serve largley to define the 14 CFR.

If the FSDO tells you something, and you base your actions on it, you have absolutely no defense if you're later determined to be in the wrong. Don't act based on what you get from the FSDO.

 

[boxcar]

Military airplanes that are considered centerline thrust are listed in AC 61.89. (The F/A-18 is on the list). This AC is published by FAA HQ. An individual FAA Inspector has zero leeway to decide whether or not a military aircraft is or is not considered centerline thrust.

 

[f18pugsley]

When I spoke to the local FSDO about removing the CL thrust restriction, he was as helpful as he could be. He showed me the pub Boxcar mentioned and asked me to show him some proof he could pass on to the proper person in OK city. The quote I have a problem with came from there.

The reason the FA-18 is listed in the pub Boxcar mentioned is because the Navy had not tested or published any SE airspeed or thrust restrictions. The Navy now publishes a Vse for the FA-18 and has tested it (with some truly eye-opening video of a departure gone bad at low altitude--Altimeter showed negative altitude over the Chesapeake Bay. We don't know how low he really was since all testing is done with the Baro set at 29.92). I supplied the FSDO with a photocopy of the appropriate pages. He forwarded it to OK city. In addition to the asinine response I received about the aoa/airspeed correlation, the rocket scientist at OK City said that the NATOPS was not a valid source because it is published by the Navy, not McD (at the time). Sorry, Mcd (now Boeing) publishes the NATOPS, the Navy only signs it to make it official.

Pugsley