Precision Approach

By lex, on October 10th, 2011

All pilots love to fly in CaVu conditions – ceiling and visibility unlimited. Unfortunately for breakers of earth’s surly bonds, the Maker of the Universe was not content to shape his creation around the preferences of aviators, and weather – ceilings and visibility in particular – can vary widely from destination to destination. If you absolutely have to get there, there are instrument approaches which are generally suitable to the task. (When there are not, the almighty generously provided for alternate airfields.)

There are basically two types of approaches aircraft use in instrument meteorological conditions to get below the weather and land; non-precision approaches and precision approaches. The former typically consist of automated bearing and range indications from a transceiver, usually – but not always – co-located with the airfield. These typically have higher approach minima for the weather conditions, depending upon the category (landing speed) of the aircraft in question, typically around 500 foot ceilings with at least 3/4 of mile horizontal visibility. Precision approaches, on the other hand, often allow a pilot to make his approach to an airfield with even lower ceilings and visibility, on the order of 200 feet and 1/2 mile visibility for single-piloted aircraft (in the Navy, at any rate). These approaches are characterized by precise course and glideslope guidance. (Non-precision approaches, on the other hand, provided courseline guidance coupled with step-down altitude assignments at defined distances from the field .)

To fly a precision approach you traditionally used an Instrument Landing System (ILS) with needles for both courseline and glideslope, or precision approach radar (PAR), wherein a ground-based controller tells his customer aircraft about his position in reference to the runway approach path. In a PAR, the controller provides mandatory heading assignments coupled with informational communications about whether the aircraft is on glideslope, slightly or above glideslope, or slightly below glideslope. You never want to be “below glideslope” and hearing that you have gone “well below glideslope” is a pretty good indicator that you’re doing it wrong, and maybe ought to consider another line of work, if you survive long enough.

More modern precision approach systems include microwave landing systems and LPV GPS approaches (localizer performance with vertical guidance), but as I have flown but a handful of the latter and exactly zero of the former, I will commit blogging heresy and stick to what I know.)

Flying a precision approach to mins is no one’s idea of fun, but the capability is important. Navy pilots are today the largest US users of the precision radar approach, the capability having vanished utterly from civilian fields and diminished significantly from the USAF inventory as well, because of the widespread proliferation of civilian ILS among both user sets, the Navy – in its unquestioned wisdom – having deployed a proprietary version of ILS to aircraft carriers and carrier aircraft home bases that is incompatible with civilian equipment.

You fly an awful lot of PARs in training, but once the wings are on and the weather fine, the general temptation is to skip the instrument practice and thunder into the overhead break. But the controllers still need training to maintain their proficiency, and so for the last week or so your correspondent has been beating the hell out of the local PAR box, when he hasn’t been thundering past an aircraft carrier at 500 feet and 0.95 Mach.

It’s been great training for me as well, the precision approach requiring a degree of, well: Precision. And the Kfir, for all its simple virtues, is nothing like a “hands off” aircraft to fly. There’s a just slightly objectionable tendency for the aircraft, especially when slow (~200 kts) to wander back forth a bit in roll, but it doesn’t matter much; the average heading tracks the assigned courseline anyway. But in pitch, especially, you’ve got to keep your eye on her or you’ll be a hundred or even a hundred and fifty feet off.

Compared to the aircraft that have comprised most of my flight time over the last several years, not to mention any professional aviator’s personal standards of precision, this is a pretty significant deviation. In a Cessna 182, Beech Duchess or even that shabby Citabria, it’s a trivial thing to tack the altimeter needle to the assigned number, but the Kfir requires a nearly obsessive focus on the vertical speed indicator, or VSI, in level flight.

So you take off heavy, and launch straight into the pattern, climbing to an assigned altitude of 4000 to get you above the local traffic. (There are three general aviation airports within a ten to fifteen mile range of Point Mugu, two of them quite busy on a given day – I had a pretty close call with a northbound Cessna 150 heading into Camarillo just the other day rolling out on final approach course.) Vectors to downwind at 250 kts, and the concentrated work begins, trimming the aircraft in both roll and pitch, with maybe a tweak or two of rudder trim.

Turning to base, you slow the jet to around 220 kts (230 or so when heavy) and prepare for the landing checklist. This checklist ought to be rather curtailed in the Kfir, since there are no flaps or other high lift devices to deploy – your main step is simply to lower the landing gear. But because any self-respecting airplane needs at least six or seven steps on its landing checklist maintains its dignity by requiring you to check hydraulic pressures, brakes, anti-skid, speedbrake position and, finally, hit the “bip” switch, transmitting a brief tone on the aux radio.

Vectoring to final is where the fun really begins, because as you “slow” to 200 kts you’re still making a whole lot better time than most aircraft in the area make in cruise. Not only do the miles click by pretty quickly, the ground controller really has to hawk the effects of his course assignments; when we first started training with them earlier last week, our ground tracks looked pretty darned serpentine.

You approach the 3 degree glideslope from below in level flight, and – so long as the controller is paying attention and has his head wrapped around your approach speed – upon intercepting the glideslope he’ll call, “approaching glideslope, begin descent” and “on glideslope”. Ease a hunk of power – not too much on the first couple of approaches – bunt the nose and, eh, voila: The VSI needle tacks itself at -1000 feet per minute rate of descent. (For reasons known only to the Kfir, the rate of descent is far easier to nail than level flight is.) If you go slightly below or above glideslope, a combination of power and attitude change adjusts your VSI to a few hundred feet above (or below) the 1000 FPM mark, and it’s important to try to lead the “on glideslope” call from the controller to keep from shooting through the other side.

Especially when heavy, you have to be careful not to get too slow on final. Below about 190 kts or so, the angle of attack to maintain your desired rate-of-descent requires a lot of power as that delta wing gets increasingly exposed to the windstream. Below about 180 or so, it becomes pretty much all the power you’ve got, and then some, the angle-of-attack tone ringing in your headset insistently.

To save wear on the tires, the practice approaches are flown to a “low approach”, almost touching down before running the throttle up to military power, raising the gear and vaulting back into the sky. As the fuel load burns down the rate of climb back to 4000 feet is giddying, the vertical speed indicator pegged out at greater than 6,00o FPM. You’ve got to ease throttle and lower the nose about a thousand feet prior to ensure that you gracefully settle on the assigned altitude rather than blowing through it.

Down about 1800 liters or so of fuel, the Kfir starts to feel a little more at ease in the pattern, and when the fuel gauge indicates 800 liters, you want to be on final for your last approach.

Low approaches are all well and good, but eventually you’ve got to put  her on deck. This has become something I can do pretty well by now, and the work is now focused on picking a spot and hitting it. Carry a few too many knots or a little too much throttle – or both – and you’ll chew up runway gliding a foot or so above the deck. Come in too slow and try to break the rate-of-descent by pulling back on the stick and the bottom drops out on you. Either way, when you do touch down you’re still whistling along the runway at between 180-170 kts, which still seems awfully quick. Once below 160, with the shortfield gear behind you, bunt the nose from the aero-braking attitude, pop the drag chute, and re-set the nose back up again to feel that gratifying tug. Just a little bit of braking gets you off the runway intersection with 4000 feet to go on the 11,000 foot runway at Point Mugu.

Which is good to know, if and when that drag chute decides to play the fool.


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Filed under Best of Neptunus Lex, by lex, Carroll "Lex" LeFon, Carroll LeFon, Flying, Lex, Neptunus Lex

One response to “Precision Approach

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

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