Aircraft have been destroyed and many aviators, and their passengers, have died as a result of the “false climb” illusion. Unlike the approach and landing illusions (See ‘Illusions during the approach and landing’) that rarely result in more misery than a hard landing or a missed approach, this one is a killer. Understanding the mechanisms behind the “false climb” illusion is quite difficult, but is an important first step in avoiding becoming one of its victims.

The false climb illusion is a classic example of the limitations of our senses, especially sight, balance, and touch, during flight. This illusion occurs when our otolith balance organs (See ‘Senses during flight’) provide misleading information to the brain and there isn’t enough information from the eyes to correct the error.

How could a healthy, command instrument rated, type experienced pilot fly a perfectly sound Beech King Air into the ground only seconds after taking off into a clear, unlit night sky? He did at Wondai, QLD, several years ago resulting in his own death and that of four of his five passengers. This accident and many, many others like it have the common features of night time departure, dark sky with no visible horizon, and unlit terrain under the take-off path.

Figure 1: The otolith organ in a ‘neutral’ position.

To understand the false climb illusion, which is also known as the somatogravic illusion, we will need first to review the workings of our ear’s balance mechanisms - the otolith organs. There are two otolith organs in each ear - one vertical and one horizontal. Their main function is to provide the brain with information about the position of the head. Each otolith contains small crystals attached to the free ends of tiny sensory hairs which are, in turn, connected to special nerve cells (Figure 1).

When the head is tilted gravity acts on the crystals and causes the sensory hairs to bend. This bending of the sensory hairs stimulates the nerve cells which then send signals to the brain (Figure 2). The brain uses the signals from all four otoliths to compute the position of the head. This process occurs very rapidly, very frequently, and without any conscious effort.

It is this resultant force that the otolith organs actually sense. The confusion, and the source of the “false climb” illusion, arises because man has evolved in an environment where gravity is the main force that influences our otolith organs. We are used to interpreting signals from the otoliths as indicating the position of our head.

Figure 2: An otolith organ when the head is tilted.

Our brain is not designed to consider whether there may be other straight line accelerations acting on our body. When forces other than gravity act on our body, usually for short duration (as when you run or jump), our brain doesn’t have to rely only on signals from the otoliths. In the majority of day-to-day situations our brain gets most of its balance and orientation information from our eyes.

Figure 4: The combined effect of gravity and straight line acceleration on the otolith organs.

Usually information from our eyes overrides that from the other balance organs including the otoliths (See ‘Senses during flight’).

In the absence of sufficient information from the eyes the presence of additional straight line accelerations can cause the brain to make incorrect head position calculations. A straight line acceleration can cause the direction in which the resultant force acts to move in a rearward direction. This causes the otolith organs to send the brain exactly the same signal as they would when the head is tilted backwards (Figure 3).

Figure 3: Straight line acceleration acting on an otolith organ (No gravity).

This is the basic mechanism behind the false climb illusion. Straight line forward acceleration causing the brain to incorrectly conclude that the head is being tilted backwards.

Now let’s look at the dark night take-off. When you’re lined up, waiting to apply power for takeoff, gravity is the only force acting on the otoliths (Figure 5).

Figure 5: The resultant force during ‘line up’. The otoliths and the eyes correctly sense the position of the head. (No straight line acceleration is acting)

They correctly signal to the brain that the head is in an essentially straight up-and-down, or erect, position. This is supported by visual information such as the positioning and perspective of runway lights and lit buildings adjacent to the runway. The otolith information to the brain is correct and is supported by visual information. No conflict exists. 

Once the throttle(s) is (are) advanced straight line acceleration begins to act on the otoliths in conjunction with gravity. The direction of the resultant force moves towards the rear and the otoliths incorrectly signal to the brain that the head is in a backward tilted position (figure 6).

Figure 6: The resultant force during the take-off ground roll (gravity and straight line acceleration). Visual information overrides the incorrect otolith signals.

However, during the take-off roll you are still able to see the runway lights as well as those of any adjacent buildings. This correct visual information overrides the incorrect otolith signals. Although a conflict exists between otolith and visual information no illusion results because the brain takes more notice of the visual information.

Once the aircraft is rotated and starts to climb the situation changes dramatically because the pilot is no longer able to see the runway lights. Gravity is still acting in its downward direction and the straight line acceleration may have increased a little as the aircraft leaves the ground and is “tidied up”. The resultant force acting on the otoliths will signal to the brain that the head is tilted backward even further. The brain knows from sensors in the neck muscles that the neck hasn’t bent and therefore deduces that the head tilt must be due to the aircraft having a higher nose attitude. In the absence of correct visual information the forward straight line acceleration during take-off results in the illusion of a higher nose attitude - a false climb illusion (Figure 7. Compare with Figure 4).

Figure 7: The resultant force acting on the otoliths during the climb-out, in the absence of correct visual information, causes a false climb illusion.

Because there is no horizon or ground lights to be seen and override the otolith signals the pilot experiences a powerful pitch-up illusion. Consciously or subconsciously the pilot applies judicious forward stick to “correct” his perceived nose high attitude. This compounds the problem by allowing the aircraft to accelerate even more causing a worsening of the illusion. Very rapidly a vicious cycle is set up with “corrections” leading to worsening of the illusion which in turn leads to further, stick forward, “corrections”.

If the illusion is not recognized, and no correct visual cue is seen, the pilot rapidly “corrects” his way into the ground. The aircraft impacts with wings level, in a nose low attitude, under full climb power, usually killing all on board.

Most people find it hard to believe that an experienced pilot could so easily fly a perfectly functioning aircraft into the ground with such dire consequences. This is because most people don’t realize how insidious and overwhelming the false climb illusion can be. It is possible, in rapidly accelerating jet aircraft for the “false climb” to continue to such an extreme that the pilot believes his aircraft is nose high vertical or even inverted when in fact it is rapidly accelerating back to earth in a progressively steeper dive.

The false climb illusion is very real and very dangerous. The best way to avoid becoming a victim of this illusion is to understand it, recognize potentially dangerous situations, and to maintain a disciplined instrument flight profile.

The otoliths convey misleading information to the brain during each and every take-off. It is, however, unusual for there to be no visual cues to override and correct the otolith misinformation. The false climb illusion only becomes a threat when there is inadequate correct visual information available - hence the term dark night take-off which is so often used when discussing this illusion.

The King Air at Wondai took off (RWY36) over unlit terrain, on a very dark moonless night. Once the pilot had rotated he would have lost all outside visual cues. Had he switched immediately and effectively to instruments and ignored his body’s sensations he may have continued an uneventful and uninterrupted climb-out. He was probably either distracted, didn’t immediately transfer to instruments, or was unable to believe his instruments. The straight line acceleration would have caused his otoliths to convey incorrect information to his brain and his subsequent “corrective” action resulted in the aircraft impacting the ground 600 metres from the end of the runway only slightly to the left of centreline. A similar series of events has claimed many aircraft throughout the world despite the illusion being recognized and well documented over forty years ago.

The other classic scenario for the false climb illusion is an overshoot flown over unlit terrain on a dark night. In a similar manner to the takeoff illusion the straight line acceleration that results from the application of full power causes the otoliths to falsely sense a nose up pitch change.

A false climb illusion will also occur with power application and acceleration during straight and level flight in Instrument Meteorological Conditions. Acceleration after unexpected entry into cloud - inadvertent Instrument Meteorological Conditions - will lead to a false climb illusion with potentially disastrous consequences for a non-instrument rated pilot.

The first step in avoiding disastrous consequences from the false climb illusion is to be especially wary of dark night departures, or overshoots, over unlit terrain (or water). It is wise to consider this illusion whenever you are using an unfamiliar runway or airfield at night.

The second step in avoiding this danger is a timely and effective transfer to instruments.

The Air Speed Indicator is the single most useful instrument during the early stages of a dark night takeoff. If VX (airspeed for the best angle of climb) is maintained with wings level you can be confident of climbing. Any speed increase above VX may indicate descent, while a lower speed will result in degraded climb performance and may put you at risk of stalling.

During instrument flight most pitch information is derived from the Attitude Indicator (Artificial Horizon) supported by the Altimeter and the Vertical Speed Indicator.

These instruments may be misleading during the early stages of a climb-out. A gyroscope driven Artificial Horizon can be subject to errors of precession when exposed to straight line acceleration. During take-off acceleration this gyroscopic precession can result in the horizon bar moving down a small distance, indicating an attitude slightly more nose high than it really is. During the initial stages of a climb-out the Altimeter and the Vertical Speed Indicator may not have had enough time to fully stabilize and could be giving unreliable readings (Less of a problem if an Inertial lead Vertical Speed Indicator is fitted).

During a take-off into a dark night it is paramount to switch to instruments as you rotate and stay During instrument flight most pitch information is derived from the Attitude Indicator (Artificial Horizon) supported by the Altimeter and the Vertical Speed Indicator.

These instruments may be misleading during the early stages of a climb-out. A gyroscope driven Artificial Horizon can be subject to errors of precession when exposed to straight line acceleration. During take-off acceleration this gyroscopic precession can result in the horizon bar moving down a small distance, indicating an attitude slightly more nose high than it really is. During the initial stages of a climb-out the Altimeter and the Vertical Speed Indicator may not have had enough time to fully stabilize and could be giving unreliable readings (Less of a problem if an Inertial lead Vertical Speed Indicator is fitted).

During a take-off into a dark night it is paramount to switch to instruments as you rotate and stay “on the dials”, maintaining VX, until you’ve reached a safe altitude. No attempt should be made to look back at the runway lights or any other ground lighting. This is extremely dangerous, especially in a low wing aircraft, because a wing may drop unnoticed as you turn your head. There is no hurry to retract flaps or undercarriage and neither a turn nor any radio procedures should be undertaken until at least 500 feet.

Remember, Night Visual Flight Rules flight is, in fact, instrument flight and the only valid reason for breaking your instrument scan is to gain navigational information from your charts or the world outside your cockpit.

In summary the false climb illusion occurs when an aircraft either takes-off or overshoots above unlit, featureless terrain into a dark, horizon less sky. The false climb illusion causes the pilot to apply stick forward “corrections” which result in a worsening of the illusion as the aircraft continues to accelerate. If the illusion -“correction” cycle is not broken the aircraft is flown into the ground.

Tragic consequences of the false climb illusion can be avoided by an effective instrument scan, maintaining Vx, and being especially careful when flying from, or into, unfamiliar runways on dark nights.