I have just seen this short video showing "NOAA hurricane hunters" flying through current hurricane "Irma".
My guess is that normally an aircraft would not be able/allowed to fly in winds sustained wind speed of 160kts, gusting to 180+kts, with the associated turbulence.
Is this aircraft modified in any special way? For example, reinforced structure, or specially designed engines?
Except for modifications related to instrumentation, not much. The WP-3D was developed from the Lockheed P-3 Orion, which was a navalized L-188 Electra.
Orion is a sturdy platform. Used during the Cold War as an antisubmarine patrol aircraft, the plane is built to take punishment.
As the NOAA noted in a Reddit AMA
The WP-3D is built like a tough old pickup truck. It can handle a lot of abuse in the storm environment and come back and fly the next day. This is mainly because of the extremely talented maintenance team at NOAA’s Aircraft Operations Center (AOC)
Simply put, it is a normal aircraft with extensive instrumentation, albeit maintained well. The same applies for the WC-130J used by the 53rd WRS:
The 53rd WRS uses the WC-130J to penetrate tropical storms. These aircraft are not reinforced in any way in fact the only differences between a WC-130J and a C-130J is the addition of two external fuel tanks (giving them longer range), a radiometer pod on the left wing and the two addition crew pallets in the cargo bay...
and of course, the instruments. Actually, the NOAA has replied to your question in their FAQs:
Why aren't NOAA's Hurricane Hunter planes torn apart in storm?
Planes are generally not destroyed by strong winds while in flight. Airliners routinely fly in jet streams with winds exceeding 150 mph over the U.S. during the winter. It's the shear, or sudden change in horizontal or vertical winds, that can destroy an aircraft, or cause its loss of control. That's why NOAA's Hurricane Hunter aircraft don't fly through tornadoes. In a like manner, NOAA pilots and crew routinely (but never casually) fly in the high-wind environment of the hurricane and don't fear it tearing the plane apart. However, they are always monitoring for "hot spots" of severe weather and shear that they can often identify on radar and avoid if it's too severe.
The simple answer to your question is no - the aircraft is not structurally modified. However, to add some context to @aeroalias's answer, here is a great visualization of windspeeds at various altitudes**. At 1000hPa pressure altitude (approximately ground level) the scene looks like this (see Irma just over the Dominican) :
Now if we go up to 500hPa pressure altitude (~18,000ft) the situation looks very different. The lower levels of the jet stream can be seen and the wind speed in the hurricane, other than being quite localized and subject to more shear, isn't really anything unusual in terms of magnitude.
It's not clear how high the NOAA plane was, but it looked to be at very least over 10k ft, I think. At 500hPa, also, we can note that the lower pressure means that the instantaneous power density of the wind is halved at the same wind speed.
By 250hPa (~34,000ft) other high level winds dominate the sky and the hurricane is little more than a mild bump:
Now, clearly there are differences between the extreme wind velocities in the jet stream and those surrounding a hurricane. Updraft, shear, and roll formations exist throughout a hurricane and these are markedly more dangerous than clear air turbulence at the jet stream boundaries. A hurricane is quite large and regular, however - its wind patterns are not so extremely changeable and this means it is not entirely unnavigable.
Arguably, large thunderstorms, for example, can be more unpredictable and dangerous to an aircraft than a hurricane because of the randomness of their interior. Not that you'd want to fly through a thunderstorm either, but at the right altitude and with sufficient care and caution it's not as mad a proposition as it may seem to embark on the NOAA's flight path through Irma.
Most aircraft can safely withstand much heavier turbulence than pilots ordinarily like to fly through. It does carry risk and it does put more stress on the airframe - surely these scientific planes get more regular and critical inspections than commuter craft simply due to the higher stresses their flights would cause - but it is clearly within the performance envelope of the plane to make such flights.
Images all sourced from https://earth.nullschool.net/. Definitely an excellent reference.
Yes one of the aircraft types is modified - to military standards, the P-3 Orion is a militarised version of the Lockheed Electra of 1957. The original P-3A had structural improvements to be able to handle military loads. From the wiki:
While based on the same design philosophy as the Lockheed L-188 Electra, the aircraft was structurally different. The aircraft had 7 feet (2.1 m) less fuselage forward of the wings with an opening bomb bay, and a more pointed nose radome, distinctive tail "stinger" for detection of submarines by magnetic anomaly detector, wing hardpoints, and other internal, external, and airframe production technique enhancements.
The other aircraft is a C-130, a military aircraft as well. Military design specifications are for transporting heavy hardware such as tanks, and landing them on a rough runway under adverse circumstances.
Constant wind speeds are the natural environment of an aircraft. The danger to the structural integrity of an aircraft is in wind gusts: a sudden change in wind speed, which bends the wings, then accelerates the aircraft which brings relief to the bending moment. The wind gust results in a lift that is momentarily greater than the weight. This is expressed in the load factor n = L/W, which is a design standard for aircraft. Military transport aircraft have a limit load very similar to transport aircraft. From MIL-A-8861:
The P-3 was a submarine patrol aircraft and designed for load factors of +3.5/-1.0, the C-130 is a utility aircraft with n = +4.0/-2.0. Very similar to transport category aircraft - the storm chaser aircraft just have to avoid the wind gusts, and are otherwise OK in the storm.
One factor that makes the P3 Orion particularly suitable for hurricane duty... this aircraft derives a good deal of its lift from the airflow across the wings generated by the propellers. This is why it has fairly small wings for the size of the aircraft - the wash from the props add lift.
That makes it somewhat more resistant to wind shear, the sudden change of wind direction that can stall an aircraft in flight by removing the airflow over the wings. If a wind shear change slows airflow over the wings on the P3, the pilots can throttle up and get some lift and controllability back.
