Viper FCF

By lex, on August 29th, 2004

After major maintenance, an aircraft has to be flown by a specially designated check pilot on a PMCF – a post-maintenance check flight, before it’s released for general use. These are also known as FCF’s, or “functional check flights.” The designation is eagerly sought out by pilots, and sparingly bestowed. Being certified an FCF pilot carries with it a mark of professional trust, and added responsibility. Too many pilots designated a check pilot in a squadron dilutes the opportunity for the select few to stay proficient in the different check procedures.

The decision on whether or not to release a jet for general use is a serious one, demanding thorough knowledge of the aircraft and its systems, and an ability to react quickly and properly to anything that may go wrong – always an elevated risk on a plane that’s had a major re-work done.

They’re also a lot of fun.

There’s generally no exhaustive pre-flight briefing to prepare and deliver (or endure, for that matter). You check the weather (it’s got be good), and local NOTAMs (notices to airmen), brief with maintenance control and jump in the jet. There’s also no endless and laborious debrief, wherein every item of tactical and administrative procedure is exhaustively analyzed for mistakes, for lessons, for learning and improving.

Depending on the kind of check flight, ground procedures vary from slightly more complicated to a huge pain in the neck. A “C”-type profile means that a major flight control actuator has been changed, something that has broken the hydraulic integrity of the system. A “B” profile means a major engine repair, perhaps even a replacement. Most laborious are “A” profiles. An A-pro has usually been down for a long time for major rework, or has had both engine and flight control work done. Every system in the aircraft is checked, even those emergency systems that are rarely, if ever used.

Since C-profiles are the shortest, they’re also the most eagerly sought after. Because you still have the full bag of gas to burn, once the maintenance check-list is done. And no one particularly cares what you do with that gas, so long as the you bring the jet back up, and don’t violate any rules.

The F-16N, which Navy pilots nicknamed the “Viper,” was probably the most fun FCF aircraft I ever flew. Hell, it was a fun jet anyway, with an enormously impressive thrust-to-weight ratio – essentially above 1:1 on take-off. During an FCF take-off it was considered important to raise the gear immediately once off the deck, level off a few feet above the runway, and then starting at around 400 knots, stand the jet on its tail at the departure end, climbing vertically to 16,000 feet in just a few moments. Zero to 400 in a mile and a quarter. Good clean fun.

Ostensibly this served to preserve options for a flame-out approach should the engine quit shortly after take-off. In practice, it closely mirrored what would be called a “low transition,” a maneuver severely frowned upon, carrying as it did connotations of unprofessionalism, of mere tom-foolery with government gear.

Be that as it may, it was not uncommon to see this sort of take-off in F-16s that hadn’t even had any engine work done.

Better safe than sorry.

Pointing straight up in a vertical ascent, you’d gently roll to align the wings with the departure heading, and start an easy pull over the top about two thousand feet prior to level-off altitude. As you approached this height, it was fun to try to time the roll out and level off so that you’d exactly recover at the assigned altitude – not too far below it, and never blowing through it.

In the Viper, the FCF procedures for a pro-C were pretty straight forward – some autopilot checks, a few bumps in all three control axes (pitch, roll and yaw), a rapid wind up, max g turn (9 g’s – ouch) and some gear/flap transitions. You could knock it out in 15 minutes or so over the Bay of Florida, and have a good hour to hour and a half of gas left to your own devices.

Depending on how you used it, of course.

Afterburner usage in a fighter increases it’s thrust by approximately 50%. But it increases fuel consumption by over 300%. This is why the F-22 Raptor (excuse me, FA-22 – right) had as one of its design specifications the ability to “supercruise,” i.e., achieve supersonic flight in basic (non-afterburner augmented) engine.

So if you were really having fun, and “lighting the cans” for any duration, you’d maybe have 15 minutes for the FCF, 15 minutes of “proficiency” time, and then have to go back and land.

So I decided one day to get my mach 2 qual.

Any long-time reader of these pages knows that my first love, when it comes to fighters, is the noble FA-18 Hornet. As I’ve mentioned before, if airplanes were women, you’d date the F-16, but marry the Hornet. It is a wonderful jet, but when it comes to top end speed, hauling-the-mail, oh-my-God, ludicrous speed, the Hornet tends towards the more sedate end of the spectrum. Built from the start as a multi-mission jet, there were several design compromises and a few of them tend to limit her Vmax (max airspeed achievable).

One last technical bit, and I’ll leave it alone: Because of the relationship of air pressure to altitude, fighters have two references for Vne. At low altitude where the air mass is much denser, this will be measured in knots indicated (or calibrated) air speed – KIAS/KCAS. At high altitude, with less air molecules to impinge upon the pito-static airspeed measuring system, it’s measured in mach numbers.

I’ll put it to you this way – all aircraft have a Vne (never exceed speed limit). The Hornet has one too, but it’s only to make her feel better about herself.

The Viper on the other hand has a Vne that the over-bold, unwary or unwise could exceed. Down low, the way we flew them (no external stores) it could pass Vne without breathing hard. Getting through mach 2.0, something that could only be done at very high altitude, took a little doing, however.

To do so requires a lot of room, a lot of gas and that the pilot be more than 30 miles to sea, pointed away from the land (so as not to break windows on the beach). The most efficient way to get through the number (that’s mach 1.0), and to keep accelerating, is to use what’s known as a Ritowski profile. It’s different for each aircraft, but essentially involves a high subsonic climb (0.97 mach or so) to around 40,000 feet, and a half-g unload (light in your seat) to get through the number while still at a relatively high altitude. Capturing that mach number and keeping it going during the next series of ascents and descents allows the pilot to build ever higher mach numbers until he gets to his goal.

It’s important not to go all the way to zero g while in full afterburner, as that tends to cavitate the fuel boost pumps, which can result in what is known to engineers as a “loss of thrust” and to a pilot as “losing the engine.” And in a supersonic F-16 at 45,000 feet, all sorts of bad things happen if you lose an engine. So watch out for that.

I got my qual one day, and found myself way the hell out away from the airfield and needing to head back. I remember being strangely reluctant – moving so high, so fast, felt like the right thing to do, to keep doing. If I turned back, I’d have to slow down, and eventually land. If I landed I’d have to get out of the jet, and walk around. If I did that, I’d be merely human again.

And part of me didn’t want to be.

But I had to, so I eased the throttle back, and began a gentle turn back towards the field, preserving altitude for the long-range, fuel conserving descent back to the field. The contrails I had left behind during my series of climbs and descents scribed white arcs in the brilliant blue sky, and looked as if they had been left by some Herculean, strangely illiterate sky writer. I wanted them to be my signature upon the heavens, I wanted them to last forever.

But they didn’t of course. Nothing ever does.

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3 Comments

Filed under Best of Neptunus Lex, by lex, Flying, Uncategorized

3 responses to “Viper FCF

  1. Pingback: Index – The Best of Neptunus Lex | The Lexicans

  2. Lex would be surprised to learn that not all aircraft have a Vne, a never exceed speed. The A-6, a product of the Grumman Ironworks, was a stout jet. The speed limit was what is termed LBA, Limit of the Basic Airframe or something like that. In other words, you could climb to altitude, point the jet straight down and shove the throttles to the stops. The Intruder would accelerate to something short of supersonic and stop accelerating right there. Ugly was strong but not slick.

  3. “I wanted them to be my signature upon the heavens, I wanted them to last forever.But they didn’t of course. Nothing ever does”.

    That was a bit of a choker to read.

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