Ask An Expert- Aviation

**Chogy** · 06 Jan 12,, 16:29

Transport aircraft are rated for a certain G load that is rarely approached in normal operations. But it is remarkably low, just 2 to 3 G. In an unusual attitude scenario, it is possible to load the airplane much higher, and the danger is bending and/or physically breaking spars and other structural members. Likewise, exceeding Vmax, red line, can also do damage. The jets that hit the WTC were well above red line... damn them for understanding kinetic energy is the square of the velocity. The cockpits were likely squawking like crazy with a thing called an overspeed clacker.

Anyway, after an upset like this, they take the aircraft apart, inspect all of the load-bearing devices, especially the wing spars and the engine struts, magnaflux for cracks, and put everything under a microscope.

These jets are well engineered and can often withstand loads much higher than their rated load. In this case, they found nothing wrong and put it back into service.

I'd guess a 737 could pull 4 G and be fine. 6 G and it'd probably break up. Just estimating.

We had a guy once save his life in an F-15 by pulling 12.5 G to avoid impact with the water, and the jet flew the next day. The same scenario in an F-16 would have resulted in a fatality, due to the G-limiting software. Needless to say, we preferred being in control over the G-available.

**Chogy** · 06 Jan 12,, 16:35

It would seem to me that considering what this plane went through, that they may of had to come up with new procedures to make absolutely certain it was safe to fly again.

To extrapolate - even without an incident, jets are regularly stripped to the bone for what are called "Heavy checks". My guess would be they did a complete heavy check and also used highly specialized devices to check for plastic deformation of the metal. No plastic (permanent) deformation, no cracks - good to go!

Most traditional guys prefer metal airplanes to composite, because science truly understands metal's behaviors under load, whereas composites are still a bit new. Composites are very stiff. They tend to not bend, but simply explode into pieces when their ultimate load is passed. But sometimes hidden flaws like delamination or improper curing can create a ticking bomb in a component.

But in the last couple of decades, huge strides in composites have been made, and I guess we have to trust the engineers that they are safe.

**Ararat** · 07 Jan 12,, 08:05

Originally Posted by Chogy

To extrapolate - even without an incident, jets are regularly stripped to the bone for what are called "Heavy checks". My guess would be they did a complete heavy check and also used highly specialized devices to check for plastic deformation of the metal. No plastic (permanent) deformation, no cracks - good to go!

Most traditional guys prefer metal airplanes to composite, because science truly understands metal's behaviors under load, whereas composites are still a bit new. Composites are very stiff. They tend to not bend, but simply explode into pieces when their ultimate load is passed. But sometimes hidden flaws like delamination or improper curing can create a ticking bomb in a component.

But in the last couple of decades, huge strides in composites have been made, and I guess we have to trust the engineers that they are safe.

Very true Chogy
Also depending if it was(or not) sustained Gs beyond catagory and design limits of the aircraft, normally a special inspection procedures is in place with the use of none destructive technics (ultrasound, Xray, dye pent, etc) will be called out by eaither the structural repair manual or a special inspection procedure much like a hard landing inspection. Also like you said I would want to depanel the aircraft and give a a good visual like heavy check (C check at least), especially if its coming due in near future anyway.

Also certain amount of damage can be acceptable if repairs are made or condition monitored (say crack growth on spar) by placing a special inspection program at certain intervals.

All Aircraft are returned to service in an airworthy and safe for flight condition regardless of if its a passenger or cargo plane or whatever.

Composite airfoils are much easier to inspect and they do not get skin buckling (like you might see on metal wings) from high G stresses. any hidden delamination can be seen by the use of xray.

**mako88sb** · 08 Jan 12,, 15:26

Thanks for the replies Chogy and Ararat. Checked into it a bit more and found this site:

Boeing 747SP Website - History for airframe# 22805

Date of the incident was 2/19/1985 and repairs and inspections where completed by 4/25/1985. Bought by Global Peace Ambassadors in 2001. Operating Certificate suspended in 7/18/2005. Likely to be scrapped due to lack of maintenance.

**Mrit91** · 17 Feb 12,, 09:59

I have a doubt:
Are there no protections against EMPs(Electro Magnetic Pulses)?
Read that EMP can render electronic devices completely useless, in this way an enemy force can break through a formidable defense just by detonating a series of EMP devices!!!

**Chogy** · 17 Feb 12,, 14:40

This is more of a physics question. Maybe someone will have an answer. I have no doubt that systems can be hardened; otherwise, a few measly stratospheric air bursts would take out an entire retaliatory missile force.

**Stitch** · 17 Feb 12,, 19:02

Yes, it is possible to "harden" electronic devices from an EMP burst, but it's fairly expensive, and usually makes the device much heavier than normal. It involves shielding the electronic components from the electromagnetic pulse using metal plates that absorb or block the pulse. One of the reasons military hardware tends to be more expensive than COTS hardware is that it is usually hardened against the effects of an EMP burst; if a COTS HDD costs $1,000, a military version would probably cost about $2,500.00.

**Mrit91** · 18 Feb 12,, 07:03

Thanks for the information Chogy and Stitch.

**Gun Boat** · 28 Mar 12,, 21:54

A question for Chogy:

Whats the fastest time you ever burnt through a tank of fuel in an F-15?

Thanks.

**Chogy** · 29 Mar 12,, 14:57

It had to be during a copper flag exercise back when copper flag was a much different exercise compared to today. Back then it was more EW-oriented, and nothing particularly exciting for any of the participants.

We launched in a two-ship out of Eglin to support either Navy or Marine (I forget which) fighters protecting the Florida coastline near Panama City. They wanted us to simply present as some sort of intruder to the domestic ADIZ. No maneuvering. We thought "F this, this is boring."

On the way out to the area to the South, one F-15 air aborted, leaving me and a back-seat rider in a D model as the sole entity. For whatever reason, we had a clean Eagle, no external fuel. External tanks are limited to mach 1.6. With the clean airplane, we had no limitations. We decided to present them with an almost impossible problem, a high & fast flyer. So at the Southern end of a slice of sky maybe 120 miles in size, we parked the throttles forward and left them there, and began a climb to 50,000' - can't go higher without a pressure suit.

The mach began to wind up... 1.6, 1.7, 1.9 - and from our own scope, we saw the defenders muff the intercept badly. The simply didn't have the time, ooomph and fuel to reach us. The machmeter cracked 2.0 by the time we approached the coast. My backseater says "Uh, dude, you're booming the coast big time." Sure enough, we were already over land. Taking the throttles out of AB caused a violent forward G of maybe 1.5, hanging us in our straps straight forward. The deceleration was impressive.

The single mach run had burned 75% of our fuel in just a few minutes, maybe 8 for the actual run. The remainder of the flight time was simply droning to and from the base. I forget, but probably 35 to 45 minutes.

An average training sortie with one centerline tank was 1.5 to 1.6 hours total.

**Stitch** · 29 Mar 12,, 17:32

EXCELLENT story, Chogy; I love reading your first-hand accounts of being an ego, I mean, Eagle driver. Not too many people these days can say they went double-sonic . . . . . .

**USSWisconsin** · 29 Mar 12,, 17:50

Originally Posted by Stitch

Yes, it is possible to "harden" electronic devices from an EMP burst, but it's fairly expensive, and usually makes the device much heavier than normal. It involves shielding the electronic components from the electromagnetic pulse using metal plates that absorb or block the pulse. One of the reasons military hardware tends to be more expensive than COTS hardware is that it is usually hardened against the effects of an EMP burst; if a COTS HDD costs $1,000, a military version would probably cost about $2,500.00.

The principle method of EMP sheilding is called a Faraday cage - it provides effective sheilding from a pulse - but has to be robust enough not to be burned out from the large currents involved. On an aircraft in flight, where can you dump that current to get rid of it? - with no ground (I would guess it needs to be directed to a HV capacitor bank).

**Gun Boat** · 30 Mar 12,, 03:43

Originally Posted by Chogy

It had to be during a copper flag exercise back when copper flag was a much different exercise compared to today. Back then it was more EW-oriented, and nothing particularly exciting for any of the participants.

We launched in a two-ship out of Eglin to support either Navy or Marine (I forget which) fighters protecting the Florida coastline near Panama City. They wanted us to simply present as some sort of intruder to the domestic ADIZ. No maneuvering. We thought "F this, this is boring."

On the way out to the area to the South, one F-15 air aborted, leaving me and a back-seat rider in a D model as the sole entity. For whatever reason, we had a clean Eagle, no external fuel. External tanks are limited to mach 1.6. With the clean airplane, we had no limitations. We decided to present them with an almost impossible problem, a high & fast flyer. So at the Southern end of a slice of sky maybe 120 miles in size, we parked the throttles forward and left them there, and began a climb to 50,000' - can't go higher without a pressure suit.

The mach began to wind up... 1.6, 1.7, 1.9 - and from our own scope, we saw the defenders muff the intercept badly. The simply didn't have the time, ooomph and fuel to reach us. The machmeter cracked 2.0 by the time we approached the coast. My backseater says "Uh, dude, you're booming the coast big time." Sure enough, we were already over land. Taking the throttles out of AB caused a violent forward G of maybe 1.5, hanging us in our straps straight forward. The deceleration was impressive.

The single mach run had burned 75% of our fuel in just a few minutes, maybe 8 for the actual run. The remainder of the flight time was simply droning to and from the base. I forget, but probably 35 to 45 minutes.

An average training sortie with one centerline tank was 1.5 to 1.6 hours total.

Fantastic.

What kind of sense of the speed do you get? Does the ship vibrate/buffet and let you know your moving above the speed of sound?

And do you miss flying those things? I would imagine there is a massive hole in your life where the F-15 use to be?

**FJV** · 02 Apr 12,, 17:49

Originally Posted by USSWisconsin

The principle method of EMP sheilding is called a Faraday cage - it provides effective sheilding from a pulse - but has to be robust enough not to be burned out from the large currents involved. On an aircraft in flight, where can you dump that current to get rid of it? - with no ground (I would guess it needs to be directed to a HV capacitor bank).

Not all shielding is created equally.

A lot depends on the frequency of the pulse you are trying to guard against.

Source:
EMI Shielding Theory - Holland Shielding Systems - EMI/RFI Shielding

The higher the frequency, the shorter the wavelength. This means that when the frequency increases, the tolerable gap dimensions decrease. In other words: doors, panels and other parts need to be connected electrically on all sides.

Source:
EMP GENERATION MECHANISM AND ITS DESTRUCTIVE EFFECT ON C3I NETWORK

The frequency band of nuclear generated EMP was found effective from 1 KHz to 100 MHz.

A 3 meter wave. That would allow for quite a large gaps in shielding.

**Chogy** · 03 Apr 12,, 20:19

Originally Posted by Gun Boat

Fantastic.

What kind of sense of the speed do you get? Does the ship vibrate/buffet and let you know your moving above the speed of sound?

And do you miss flying those things? I would imagine there is a massive hole in your life where the F-15 use to be?

At altitude, very little sense of speed. No real noise, no added vibration, it's all very smooth. Breaking the sound barrier in a modern jet is a non-event. About the only clue is the old analogue (clock) airspeed indicators almost always hiccup, or blip, as the shockwave separates and the airflow is momentarily disturbed. It's nothing at all like the old stories from the Yeager days.

It was a good time in my life, but it's definitely a young man's game. Experience cannot always make up for the physical demands and the reaction speed that youth can handle much more effectively.

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