NOTE: It cannot be emphasized enough that tampering with air navigation equipment, or in any way interfering with the safe flight of aircraft by transmitting spurious signals or otherwise, is a Federal crime subject to the most severe penalties. In no way is the following to be construed as advocating, facilitating or encouraging any criminal act against aircraft or airports. This information is provided SOLELY for the purpose of discussion in the context of suspicious accidents involving aircraft, and to demonstrate the simplicity of creating aircraft "accidents". Technical details that would allow construction or operation of practical devices to cause aircraft crashes have very purposely NOT been detailed, and certain essential elements of the information below have been deliberately skewed to be misleading. Note, however, that these distortions do not alter the concept.

First a brief tutorial:

In conditions of low ceiling and/or low visibility, pilots rely on instruments to keep them informed of an aircraft's altitude, attitude, air speed, rate of climb or descent, etc. This is most important when shooting an approach in high-performance aircraft under poor weather conditions, when small errors can have tragic consequences.

Icing on airfoil surfaces can greatly influence a plane's stall speed - an important consideration on final approach - but modern stall warning indicators normally give ample warning of this sort of problem and prompt the pilot to apply power before airspeed drops below critical level. Most pilots practice stall-recovery on such a regular basis that corrective measures are a natural reaction. If this were to occur during approach, the pilot might decide to land provided there were time to recover proper airspeed and control, or to abort the approach and go around again.

When making a controlled descent through cloud cover, the altimeter and rate of descent indicator are all-important, since a pilot cannot (always) see the ground until he descends clear of the cloud cover or ceiling. Two types of altimeters are in common use, the "manual" type and the "radio" type. Both rely on the instrument being set to the correct barometric pressure so as to indicate altitude accurately.

Altimeters:

Manual altimeters (common on small single-engine aircraft) are adjusted by turning a knob to calibrate the altimeter to the local barometric pressure. This is done before takeoff by setting the instrument to the known runway elevation and may be readjusted for changing conditions enroute by monitoring weather frequencies for local barometer readings. Every novice pilot learns the rhyme "Low to high, you're flying high. High to low, look out below." This reflects the fact that when flying from an area of low pressure into a higher pressure zone, the altimeter will indicate a lower-than-actual altitude, meaning the aircraft will be higher than the altimeter indicates. When flying from a high pressure area into a lower-pressure one, the reverse is true and the aircraft will be lower than the altimeter reading. There have been many accidents due to failure to properly maintain correct altimeter settings. Such accidents most often occur under IFR conditions in mountainous terrain and on approach to landing.

Radio-altimeters (more common on high-performance and multi-engine aircraft) are "self-adjusting" in the sense that they receive coded barometric pressure signals transmitted from area service centers, and automatically apply the necessary correction on a regular basis.

Approach Navigation:

Several Instrument Approach systems are in common use at general aviation airports. Disregarding the highly sophisticated systems found at large commercial or hub airports, approach systems may be categorized as "Non-Precision Instrument" (NPI) and "Precision Instrument" (PI) systems. Simply put, both types provide pilots with information on the orientation of their aircraft relative to the runway threshold, in terms of heading or course as well as approach slope or glide angle. NPI systems typically rely on a Non-Directional Beacon (NDB) and high intensity lights as well as other visual aids such as the "Precision Approach Position Indicator" (PAPI). In addition to lighting and visual aids, Precision Instrument approaches incorporate electronic "NavAids" such as radio position markers and an electronic Glideslope transmitter that provides a directional "beam" to guide the aircraft in its descent to the runway. Full Instrument Landing Systems (ILS) may include even more sophisticated position locating equipment.

Effects Of False Signals:

1) Altimeter error:

As noted above, radio altimeters incorporate very-high-frequency (VHF) radio receivers that allow them to continuously re-calibrate themselves on the basis of encoded barometric information. However, reception is only possible at altitudes high enough to afford a "line-of-sight" to the transmitter. As an aircraft descends to lower altitudes, the line-of-sight becomes less and signal strength is reduced. When the signal is finally lost, the radio altimeter remains set to its last calibration, which in normal circumstances is completely accurate for the area of operation.

If a "false" altimeter transmitter is set up somewhere in the vicinity of the landing zone, its signal will overpower and ultimately replace the true signal as the aircraft descends. A deadly situation is created if the encoded signal is such as to cause the aircraft's radio altimeter to gradually recalibrate itself to a much lower barometric reference point, since this will have the effect of indicating a much higher-than-actual altitude. In low ceiling conditions at NPI airports not equipped with an electronic Glideslope (such as Eveleth-Virginia), a pilot may thus be made to think he is hundreds of feet higher than he actually is. If the ceiling is only 200 feet or so (as it was at Eveleth), the pilot would only become aware of the error when he broke through the cloud base at 200 feet, at which altitude - and at a descent rate predicated on a higher altitude relative to the runway end - it would be too late to correct without stalling and crashing the aircraft.

This type of "sabotage" would be extraordinarily simple to effect for anyone with a knowledge of radio transmitters, would not require disabling any air navigation equipment, and could make use of a recorded or computer-generated encoded datastream.

2) NavAid error:

It is also possible to position a "false" Glideslope transmitter a mile or so from the runway end and simultaneously disable the real one. In conditions of poor visibility, this could cause an aircraft to "land" catastrophically short of the runway. If Marker Beacon antennae were similarly relocated, an aircraft could be totally disoriented. It was exactly this kind of sabotage that was implicated in the crash of a junbo-jet on a far-east mountainside ten or fifteen years ago. The details escape me now, but the case attracted attention because some high officials of something or other were killed. It was theorized at the time that a glideslope transmitter had been mounted on a military jeep parked in the jungle at some distance and at right angles to the runway. When the pilot reached what he thought was the marker, he turned and began his descent - right into the mountainside.

This kind of thing would require considerably more organization than the scenario in 1) above, but it's certainly feasible for those with the motivation and skill, not to mention those with ready access to the necessary technical toys.