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Why Do Military Helicopters Fly So Low?


Have you been trying to read peacefully but roaring helicopters flying too low keep distracting you? Are you an irked farmer whose chickens have literally been scared to death by the extremely loud ‘whop-whop’ sounds of these machines? Maybe you’re just an aviation enthusiast who gets fascinated by the amount of detail you can see when a helicopter flies abnormally low.

Military helicopters fly low when training for wartime operations. Flying low allows helicopters to reduce the noise they make and fly under the scan of enemy radar. Flying low is dangerous and takes lots of training and practice. It is this practice that has them flying low in your area.

if your home is in a location that proves the perfect terrain for this type of flight training n you can be unlucky or lucky depending on your love of aviation. Although most military bases try to conduct low-level flight training in designated areas, there may be times when routes take them over your house.

The two main reasons for flying low are:

  1. Detection Avoidance
  2. Surprise Tactics

Let’s take a look at both of these and see how they relate to the helicopters thundering over your home…

How Do Military Helicopters Avoid Detection?

Military helicopters use terrain to absorb the sound made by them. By flying in valleys, rivers, and between trees pilots can reduce how far the sound travels. Low-level flight also keeps helicopters under the scan of enemy radar due to the curvature of the earth and how low the scan can sweep.

The element of surprise is one of the greatest advantages in battle, and because of this military leaders look at every aspect of surprise whether it be stealth materials and technologies, or procedures and tactics to take advantage of the terrain within the theatre of operations.

By far the most common way that military helicopters are detected is by radar. Ground-based enemy radar sites use radio waves to look for approaching aircraft

To better understand how flying low can help evade these systems, let’s look at how these systems work

How Does Radar Work?

Radar is a system used to detect the location, distance and speed of aircraft, ships and other objects using electromagnetic pulses radiated out from the device.

The ground-based equipment sends out short bursts of radio signals in predetermined directions towards the sky. These radio beams hit the metallic surfaces of aircraft and get reflected back to the ground-based radar station, where they are received by the receiver in the radar equipment. These reflected signals are referred to as echos.

The radar system calculates how long it has taken a certain pulse to be received back as an echo. Since the speed of the radio wave is a known constant, the distance of the aircraft from the equipment can be calculated. Direction is determined by the magnetic heading the signal was sent out.

How Do Helicopters Avoid Enemy Radar?

There are two ways for aircraft to avoid enemy radar. Position the aircraft outside of the radar scan or use materials that absorb the radar signals and allow minimal or no radar signal to bounce back to the radar receiver. Depending on the sophistication of the aircraft it may use both techniques.

Radio waves emitted by the Radar are absorbed and reflected by terrain, buildings or obstructions, the lowest beams are those sent parallel to the ground, the rest are sent up into the sky.

As a radar beam travels parallel to the ground from its point of origin, it gets farther away from the ground as the ground continues to dip away from it due to the earth’s curvature. This creates a region below the beam called the ‘Radar Shadow’ which gets bigger as the distance from the radar increases.  Detection of aircraft in the radar shadow region by the radar is virtually impossible.

Military helicopters take advantage of this shadow by using nap-of-the-earth (NOE) navigation, which is sometimes called “flying under the radar”. NOE navigation involves flying as close as possible to the ground. It is this technique that pilots are practicing when flying low around your home.

NOE navigation in the radar shadow region gives a helicopter the advantage of the element of surprise as it can get close enough to the enemy, attack, and flee before the target detects it early enough to counter using Surface-to-Air Missiles (SAM) and Anti-Aircraft Warfare (AAA).

Radar systems depend on a clear line of sight to the helicopter for it to be detected. If the helicopter cannot stay in the shadow the pilots will use the terrain. Radar is unable to penetrate mountainsides, buildings, and dense forest.

The pilots can then use these ‘Obstructions’ to mask themselves from the radar’s line of sight when in radar range by flying through valleys, around hills instead of over them, and even below tree levels where there’s a clear path between them.

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How Do Military Helicopters Use Surprise Tactics?

Military helicopters like the AH-64 Apache and OH-58 Kiowa use radar and imaging systems mounted above the main rotor systems to allow the helicopter to remain hidden while surveying the area. Only a few seconds are needed to scan and capture enemy targets by the radar and imaging systems.

The Longbow Radar mounted above the main rotor system on the AH-64 Apache is capable of tracking up to 128 targets simultaneously and engaging 16 targets at once within a quick 30-second scan. By having the main fuselage of the helicopter hidden behind an obstruction it allows for very effective surprise attacks.

AH-64 Apache with its Lonbow Radar Mounted above the Main Rotor System

To train for this pilots must fly low and fast to their area of observation and find an area to hide and peek. It is during these missions that you may see a helicopter flying really low!

The other type of helicopter with an above rotor system is the OH-58 Kiowa. Unlike the targeting system of the Longbow Radar the Kiowa is a reconnaissance-based helicopter. In its ‘Beach Ball’ is mounted a gyro-stabilized optical camera, thermal imaging camera, and a laser range finder/designator.

OH-58 Kiowa Reconnaissance Helicopter

By staying hidden from sight the pilots can use the Kiowa to provide eyeballs to commanding officers of potential threats out in the theater. Again, the pilots of the Kiowa need to fly in low and fast to avoid detection.

What are the Dangers of Military Helicopters Flying Low?

Flying a military helicopter low dramatically increases the risks of collisions with terrain, towers, cable spans, birds, and trees The reaction to deal with an immediate emergency is very quick and it requires intense practice and concentration. Regular training and known routes reduce risk.

No matter what aircraft a pilot is flying, the closer to the ground they fly, the higher the risks. It is for this reason why the military demands rigorous training for low-level flight for its pilots. Flying low and fast is one thing, doing it in a warzone while trying to remain undetected is a whole other ballgame.

Here are just a few of the common things pilots are trained to be aware of when practicing and operating low-level flight operations:

  • Cable spans are near impossible to spot in the air. Even though military pilots are taught to scan for wires by looking for pylons, it may be too late by the time they spot one. Routes are surveyed for cable spans before being flown at speed for training.
  • Birds generally fly up to 2000 feet, especially when migrating. Contact with a bird anything bigger than a pigeon can cause damage to critical components like the rotor or engine, or come through the windshield injuring the pilot. Low and high-speed birdstrikes are very serious.
  • Low recovery time or little time to initiate a forced/emergency landing in case of emergencies such as power loss or stall
  • Thermal turbulence and mechanical turbulence caused by air blowing against structures such as buildings may lead to momentary loss of control.
  • Windshear can cause a sudden loss in altitude or airspeed which is dangerous at low levels. Microbursts are a type of wind shear that occurs near the ground.
  • Controlled Flight Into Terrain (CFIT) – this is the unintentional crashing into terrain with an aircraft that is under positive pilot control. Reduced visibility or momentary pilot distraction are common causes.

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Rick James

I am an aviation nut! I'm an ATP-rated helicopter pilot & former flight instructor with over 3500 hours spanning 3 countries and many different flying jobs. I love aviation and everything about it. I use these articles to pass on cool facts and information to you whether you are a pilot or just love aviation too! If you want to know more about me, just click on my picture!

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